There is a lot that the Bible doesn’t cover explicitly. The Bible would have to be the same area of a football field and a mile high in order to cover everything that every human could possibly want to know. Good luck reading that. :)
There are somethings that can only be deduced by reasonable reasoning. Is it more reasonable that Cain’s wife come from a different species or is it more reasonable to deduce that Cain might not have been the only child of Adam to have a problem with the requirements of God and acted on their wayward desire?
You left out the possibility of God creating other people who weren’t mentioned specifically.
There were other races of people in the world. God spoke to Israelites and was all about His chosen land and people. Matter of fact I think it’s even at the least alluded to that us gentiles were grafted in but only due to Israel’s stubborness and disobedience. Genesis can be interpreted as two separate creations, with Adam and Eve being the focus because they were to be the bloodline of which the savior would come. There are some who believe the “Beasts of the field” were in fact, not animals. I won’t repeat the rest here as that particular viewpoint could offend. I read a long essay about it years ago.