My take on syscourse based on my own experience and seeing arguments and anecdotes from both sides, along with reading some medicine papers.
My relevant history (optional if you don't want to know where my opinions come from) (ALSO THIS PART GETS EXTREMELY HEAVY, POTENTIALLY TRIGGERING, scroll past this part if it makes you uncomfortable)
To make things clear and to make you aware of any bias I might have, I have to tell you a little bit about my history. I've sometimes heard thoughts that talk to me/comment on me that were distinctly different from my conscious thoughts for as long as I've remember. I remember a few instances where I was overcome with strong feelings and my body seemed to move with zero output from me, but far more common is memories where the feeling becomes overwhelming before the memory cuts off completely. These relatively short gaps in memory happen every now and then even during my teenage years. When I was under 8 years old I had a lot of night terrors (almost every night I had at least one) and those night terrors continued to play out as I woke up from them. They seemingly continued in the space I was in, even when I went from one room to the next. They only ever faded at the time when my parents were able to calm me down. Fast forward to the point I turn 13 and I start to get very detailed, vivid dreams about escaping from something in a landscape that looks like a battle field after most of the battle is over. The dream ending each time I'm caught and I see the face of my pursuer, the reason why every corpse I had time to look at had a big hole in their chest. And when I woke up after the third time of seeing such dream I realized that my pursuer was me. Coupled with the times I felt something which I could only describe as deep, undiscriminating bloodlust surfacing in certain situations, I thought I was becoming evil. The only reason I didn't harm anyone when it surfaced was that I was able to tense and lock every single muscle from my limbs so that whatever it was that wanted to kill and hurt people around me could not move. After a very gruelling summer I felt this something fade away after I saw a music video on TV of all things. I relaxed for the first time in months.
Then, all of the other problems with my head started rising. I went to therapy in the following fall but I didn't get answers for years. I started progressively get worse and worse, especially as school got more challenging and stressful. I started with anxiety, then panic attacks then i got paranoid that people would harm me very severely if they knew who I really was and what I really thought.
One of my old night terrors re-surfaced. It involved getting locked into a dark room with absolutely no light, and with darkness different monsters started to show their faces. In the dream I was banging on the door begging my father to let me out, promising that I would be good and forget everything. Eventually the door opens, my father pulls me in to an embrace, makes me promise again that I will forget, takes out something that works as a pendulum, tells me to look at it and tells me to forget everything as the bell rings. There's a chiming sound and the dream ends.
I also start to have pretty solid conversations with the voices that have commented on my life every now and then, and I get the feeling that I'm talking to a different side of myself. I start to rely on what they say to other people and with their encouragement, even give them the control of my body. When I do that those memories become distant and fuzzy, similar feeling of remembering a mundane dream.
So when I'm stepping into adulthood, my relationship with my family is pretty much falling apart. I move out. For a while I get better but then the paranoia strikes again and I start getting very severe depression symptoms to boot. It escalates until one morning I wake up and see blood everywhere. Then I see different monsters following me among the dripping blood. It stays like this for 3 days straight until I give in and call the urgent care in my city and get admitted to a hospital.
A year and a half later I get a diagnosis of schizophrenia. Half a year after that and I finally trust my therapist enough to open up about the fact that I don't think I'm not alone in my brain. By this time, we have established that I do not experience auditory hallucinations, just visual or tactile ones. Delusions are another game. However, my therapist seems to believe that this is my experience but doesn't seem to believe that these other people are different from me in a significant way. We are still trying to explain what is the same and what is different to her. I'm also informed that while I do display dissociative symptoms, I do not need the diagnosis for legal reasons because I already have a severe diagnosis that justifies the same support and benefits I can legally get in my country. I'm also told that because of my schizophrenia diagnosis, getting diagnosed with any dissociative disorder would be so much more difficult. As such, I do not have a diagnosis for dissociative disorder of any kind nor will I be getting one soon. Also, because of our system nowadays works quite seamlessly together in every day situations, my workers don't see any reason to treat it either.
There is a clusterfuck of other mental illness symptoms I've experienced during the years as well, but they aren't related to this discussion.
My current take on syscourse (shortened version)
While I do believe that systems can be formed in a myriad of ways, I personally believe that DID/OSDD systems do stem from trauma. There are other ways to "acquire" alters than dissociative reaction to trauma, even for those who have experienced trauma, but those require conscious effort as far as I know. In addition, sometimes the effect of trauma is indirect, but still very much the reason for why an alter is formed (for example, introjects that represent some traits that already existing alter(s) find "cool" or "ideal" or "admirable", traits that previously existing one(s) want to embody themselves but can't internalise properly because of the trauma has made them believe those attributes can't possibly be theirs.) (Not all introjects are formed this way, obviously, but I've gotten the impression from other people that this can and does happen.)
Also, because of the nature of DID and OSDD it is entirely possible that most (or the most active) of the system members do not remember anything traumatic happening, ever. Or if they do, they do not remember the distress related to it, or they might even think that their distress is not "severe enough" for it to be a part of their system forming. That's kinda the whole point of trauma related dissociative disorders, just saying.
Reasons for my current takes (Aka my current takes, detailed version)
1. DID/OSDD stems from trauma. I have yet to find any research papers that has come to other results or even speculates that there could be another reason for it. If you have access to such papers, please do send them my way. The reason why trauma is not a requirement for such diagnosis in all places is probably due to the fact that dissociative disorders by nature are ways for the brain to avoid anything that could be devastating as a survival mechanism. Case in point: for the longest time I didn't think I could have DID/OSDD despite being aware of being a part of a system (admitting that in itself took years too) because I didn't think I experienced anything traumatic. Then I thought that yeah, I experienced something traumatic but it wasn't THAT traumatic. Then I thought that my reoccuring night terror was actually a memory and that's how we became we, which made me think that maybe something bad(tm) happened but it was suppressed artificially and I wouldn't be "a part of a whole" without it. Only lately I've accepted that my parents divorce and emotional neglect as I was under 5 years old were probably traumatic enough when combined with other little things for a traumagenic system to be formed. Especially because I didn't get any meaningful help and support from adults around me, leading me to compartmentalise my feelings about things into "appropriate" and "not appropriate, need to hide and destroy" categories.
2. Systems can be endogenic in origin. I mean it is possible to cause dissociation on purpose (though it's rare), but it's also possible to imagine something so strongly that it seems to gain consciousness/change on it's own after some time, even without any brain oddities present. However, as stated above, this requires conscious effort. This effect can be seen most commonly in writers when they think about "How would character A or B react to things?" to the point that eventually the characters "seem to write themselves". It is my understanding that this phenomena can be taken to the extreme, so much so that the person who created this thought form doesn't know if they can stop the thought form even if they needed to. And at that point they are a system. Not a DID or an OSDD system, as those are formed subconsciously/involuntarily as a way to survive, but they are no longer alone in their head, hence, they are a system. I also believe that creating endogenic systems comes more naturally to some than others and that adding new alters becomes more and more easy the more one does it, to the point that it can be half accidental eventually. That still doesn't mean that one could become an endogenic system by accident.
And yes, becoming an endogenic system to help one cope with life is definitelya thing, but it still doesn't mean that becoming a system is something they would not survive without or that they wouldn't need conscious effort for it.
Again, if you have scientific resources saying otherwise, please send them my way.
Concerning the term "tulpa" (TLDR on the bottom of the post)
The name of tulpa comes from a very specific religion. It is a highly advanced practice for that religion and while rare, I have come across claims that it is still practiced by very experienced members of that religion and is still something that is revered and sacred. Unlike some of you seem to believe, there are different types of Buddhism with different requirements, just as there are different types and sects to every religion ever practiced on Earth. As someone who has strong ties to a different closed practice which does closely involve a commonly-thougth-to-be-dead religion, something quite few people with similar ties to the practice think too, I personally can't condone attempts to practice something you have only a surface level of understanding about. Even if you're genuinely intrigued by and interested in the practice, doing so without going through proper channels (that are there to ensure you understand the significance and methods btw) is cultural appropriation.
It becomes even more severe when you don't even brother trying to go through appropriate channels (if those channels don't exist, there's a reason for it!!!!) and just... Try to wing it with whatever watered down information you come across being told from random person to another random person. It's ignorant at best, and being willfully disrespectful is not too far behind. You can not know the significance of the practice that way, and if you do any kind of spiritual practice and you happen to get it wrong, it could turn out to be dangerous/detrimental for your wellbeing in the worst case scenario. This risk is mitigated when you know what to do if you make a mistake, but there's no guarantee that you can find the right course of action after those mistakes if you haven't been going through proper channels.
As for the terminology itself, in cultural and religious practices, names have a significance. They can be extremely sacred. Even if the end result is in your eyes similar enough ("soup is soup") the methods themselves are usually an extremely important part of the process and just as sacred. Calling something by it's name is attaching all those sacred practices and significance to it. The name carries association and power someone from outside wouldn't even dream about. I personally don't know the significance of this term, nor what it really actually means so I don't use it when talking about things not related to this specific practice. Even if a name of a religious practice becomes common outside of that culture, it doesn't mean that the name doesn't still carry all that meaning with it. Claiming otherwise is the equivalent of trying to strip the whole practice of it's meaning.
This form of appropriation is sadly common concerning certain religions, traditions and practices. It still doesn't make it okay to do. It wasn't too long ago that native american traditions and clothing were comonly acceptable to mimic and copy to the masses without much care for their significance and meaning, but it was still disrespectful and wrong and racist tbh. Why would appropriating this tradition be any different?
Use another term for things that (to you) seem similar to another cultures tradition but definitely are not part of that tradition. For example: energy centre vs chakra, thought form vs tulpa, honouring the bear you killed vs peijaiset etc...
It's disrespectful to do anything less if you're even slightly aware that there's a possibility of a difference. And now you are aware of it.