- 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.
I do remember reading somewhere that smartphones don’t have a standardized bootloader like PCs do with BIOS and UEFI, and that it can vary between manufacturers and devices. Could be wrong about it. Also some manufacturers really don’t want you to be able to install custom software, like Samdung in North America. If you buy a Samdung device in NA…even if it is carrier unlocked… the bootloader will be impossible to unlock.
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