Because the hardware is usually locked down much more than a typical PC and there is far less documentation. So the barrier to entry is substantially higher than for OS development on PCs.
That's what I am asking for. Is there really no documents for the hardware on android phones?
I am not asking for any particular phone or a particular combination of chips used, but at least an overview document but tells in details about the hardware scenario of the mobile platform.
Sure, but you'll be signing a lot of NDAs to get access.
what about pine phone?
That might work. Though you probably aren't going to be able to access the baseband processor without some blob.