- Unlock bootloader (depending on vendor, you have to do an online verification),
- flash a recovery.img,
- load into recovery mode (which, depending on the phone, might need extra work)
- wipe some caches,
- select new os/rom image,
- pray it doesn’t brick your phone.
You’d think someone would’ve learned a thing or two from the easy graphical installations linux and even windows have been offering since the late 2000s.
Look up the history of how Android was developed, particularly what happened when Google took control, and you will find your answer. Ofc if you want to help, feel free to be the change that you want to see in the world, I’m only talking here about the past, as you asked about.
TLDR version of that history: greedy corporations surprised everyone, somehow, by ultimately acting in a greedy manner. B-b-but their slogan was “Don’t be evil”! - once upon a time. And ofc Apple sure as hell wasn’t going to cooperate with that “nonsense” of allowing anyone to take even one step outside of their walled garden. So what then is left to us, besides what you see before you?