- 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.
Less tinfoil in my take: a good reason there doesn’t exist other OSs is a lack of drivers and support for hardware. Good luck getting your screen to work if it’s proprietary to the manufacturer and device. Can’t communicate with the ASICs if they don’t use standard protocols, etc.
This was a big issue back when I was involved in the LG G Flex 2 community and I can assume it only got worse since then.
PCs are a bit different IMO since generic drivers might get you pretty far. Even then, support for modern graphics cards (for example) would be near impossible without the manufacturers playing nice or in-depth insider knowledge