Following on from that last post (and a little bit the one before that), I wanna talk about tech systems
In the way that a magic system is the way that magic in a fantasy story works, a tech system would be how the (specifically fictional) technology in a sci-fi story works. You're unlikely to need to actually invent the things in your tech system, but the more you understand about how this tech works in your story the more you understand what it actually can or can't do and how its function might influence its design
So a rough guide for how to develop a tech system for your sci-fi story would be something like this:
Pick a non-existent technology that will exist in your setting. Maybe this is part of your central idea or maybe it's something you want to explore the potential of or maybe it's something that has to exist for the story to work. For example, if your story breaks unless the characters can send messages between star systems near-instantly, you'll need some sort of FTL Communication
Come up with an outline of how that technology works. How detailed that outline should be is something you'll really need to decide on a case by case basis but it's a good idea to consider how close it is to things we actually understand. For our FTL Communication example we might say that it uses quantum entanglement to teleport photons between places
Work out the "prerequisites" this technology requires. For example, in real life the wheel as a component of vehicles seems to have been adapted from a potter's wheel, which in turn postdates the development of pottery, so pottery and potter's wheels could be considered "prerequisites" for the wheel. Back to the FTL Communications, the big problem with using quantum entanglement this way is that the no-cloning theorem prevents it, so to go ahead with this you'd need to say that scientists in your setting discovered some sort of fictional loophole or workaround or something to get around the no-cloning theorem - I'd recommend having this be something incredibly complicated and difficult but not deciding what it actually is. This also points to a much more in-depth understanding of quantum physics that we currently have
Work out the implications of this technology and its prerequisites. Wheeled vehicles, especially paired with domesticated horses, shortened travel times, increased the possible area a polity could cover, and made raids and tribute extraction easier, among other impacts, while pottery itself is a pretty big fucking deal for a lot of reasons. Meanwhile, if you can send information FTL, can you do the same with matter or is that in the experimental stages or outright impossible for whatever reason? If this world's knowledge of quantum physics is so far ahead of ours that they can get around the no-cloning theorem, what other speculative or downright impossible quantum physics things do they have? I'd recommend contacting someone who actually knows quantum physics, explaining why you're contacting them, asking if they'd be happy to answer your quantum questions, and if they say yes asking about things like what else the no-cloning theorem prevents or what applications various speculative quantum ideas might have
Design-wise, this FTL Communications system sounds like it fires photons at a machine doing a lot of complicated quantum physics (transmitter) while elsewhere it detects photons emitted by a very similar machine doing a lot of complicated quantum physics (receiver). My immediate thoughts are using quantum supercomputers for an idea of how the complicated quantum physics machines might look (since quantum supercomputers are complicated quantum physics machines), basing the other part of the transmitter on radio transmitters, lasers, and X-Ray machines, and using an antenna or satellite dish for the other part of the receiver. That is my off-the-top-of-my-head thought but if I was actually doing it I'd want to track down some diagrams of quantum supercomputers to figure out which bits do what, reading up on radio equipment and similar tech, and then putting some more thought into it
