- 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.
Adding to the “no standard bootloader” point: you can’t boot your typical smartphone from a DVD or USB stick like you do for a PC. The procedure to flash a new rom is probably meant only for recovery purposes by tech support, rather than the end user