What is your headcanon for monster reproduction?
Oooooh boy. How do I explain this other than just, magic?
Like... it's somewhere in between @askfriskandcompany 's explanation, @catboymoments 's head canon with some other ideas sprinkled in? Uhhh... More under the cut because this gets a bit in detail and I'm not sure if everyone wants to read about that?
Alright... So in Ish, I try to think of it as close to human reproduction as possible. Transfer of cells during a very intimate and close ritual between two people who (hopefully at least) love each other. Just with magic involved. This is because if I don't start with that base, I get very confused. Even looking closer than that simple base, I get confused. Alright? Alright.
So, transfer of cells. Most of y'all's middle school and high school health classes should've taught you that babies happen when a sperm cell (from dad) fertilizes an egg cell (from mom), which then grows into a baby. However with monsters, we're able to have two dads or two moms or two whatever because Petra loves making fan kids and loves the fandom's general acceptance of homosexual monster couples being able to have biological off spring and you can rip this from my cold dead hands. With monsters, it doesn't really matter what the cells are called, or which parent they come from, just that they work much the same as sperm and eggs except these are made of magic. Lotta things about monsters are magic, so don't question it, okay?
Because not all monsters have the same genitalia as humans, and in fact almost all of them have very varied and different genitalia to humans (if they have any at all), the organ involved in this transfer of cells is the soul. During the intimate ritual (be that sex or a special and ancient spell), both partners' souls release the cells, which collide to make a sorta magical fertilized seed. Once that seed is formed, unless both partners determined who should be the one to carry before hand, the seed is going to gravitate to the parent who's body and soul are most adept and capable of carrying. Let's take Sans and Toriel as an example. Sariel was an accident, she was not planned. Her seed gravitated to Toriel not only because Toriel is a boss monster, with more humanoid anatomy and organs like a permanent womb, but also because in general, Toriel is in much better health than Sans. She has a stronger soul and a healthier body.
But what about the zygote's development? I mentioned that homosexual monster couples can have biological kids, how does that work when two gay male monsters don't have wombs. Or even when any monster couple doesn't have one with a womb (like with skeletons). Well that's were we get even more confusing and hand wavy with the maaaaaaaaaaaagic explanation! While it may very from monster to monster, for the first bit of the pregnancy, the carrier carries the monster baby seed in their soul as it starts to undergo mitosis and form a zygote of both physical and magical cells, often seen as a bunch of floating dust.
After the carrier's body has had time enough to form a womb thanks to magic mumbo jumbo and/or the zygote becomes an embryo at around the two week mark, the now embryo continues it's growth in the carrier's womb, still connected to it's parent's soul as it's own begins to form. The baby's soul doesn't finish fully forming until the baby is ready to be born after 9 months or gestation, however it does form into what is known as a proto-soul after 16 or so weeks when the little blob of magic forms into the recognizable heart shape of an Undertale soul. A similar process can be seen in individuals from worlds which do not have physical manifestations of souls who spend long enough in Ish to experience the Universal Assimilation Effect, except for them, they're souls form fully after the blob stage. Only having to go through a bit more adjustment as a trait is determined. For a more exact comparison, Exriel/Flowey's soul under went a process similar do how souls are formed with babies, proto-soul stage and all. This process with the soul formation occurs with all babies in Ish, not just monsters, and the baby and parent's souls remain linked until the baby is born.
Okay, but what about monster and human couples. Monsters can reproduce with humans too, how the hell does that work? Well, much the same it works with monster loving monster relationships, just with a little bit more fudging. The transfer still happens through the soul, and the seed still goes to the more capable body and soul. Which often tends to be the human parent, unless the human parent doesn't have a womb. Let's say we have a monster human couple, where the dad is a monster, and the mom is a human. Seed's gonna go for mom most the time. Even if her soul may not be as magically adept to carry the seed during the short zygote phase (unless she's a wizard or some kind), Mommy's body has a permanent womb. If the dad is the human though, seed's gonna go for the other, monster parent, whether that parent be a mom or a dad or whatever, because human daddy can't magic his body to form a womb. Does that make sense?
I really hope it does, because that's the best I can explain it. If you have any further questions then feel free to ask.