- 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.
A few years ago, there were huge issues with reseller unlocking the bootloader to inject ads on the phones they sold, which forced many android phone manufacturers to add online verification with long wait time to prevent bulk unlock.
That may be an excuse they used, but I doubt that was really their motivation.