Even Tolkien was not able to satisfactorily answer fundamental questions like the origin of orcs.
He talks about it in one of his Letters - and says that’s why he ended up having to reduce the amount of “orc talk” because it was making them sympathetic and he didn’t really know how to handle it - for the story he wanted generic disposable “baddies”.
That explanation does not work in Tolkien's universe though. Can orcs be redeemed? Do they still have a soul? What happens to it when they die? Do they go to the halls of Mandos? Can they be reincarnated like elves?
Tolkien was never satisfied with any explanation because he could not fit them inside his world's theology, which is inspired by Christianity but does not match it one to one.
The only pity is that J.R.R. Tolkien never finished it, so some things like where Hobbits came from or where the Blue Wizards went remain (mostly) unaddressed.
The extended LOTR film depiction of them being made from a mud-like mixture is nonsense.