Linux Phones

For one specific phone? Might be fairly simple and quick for a team that knows what it's doing.

For several or even more popular phones? Here we run into issues. Because smartphones (and most other consumer ARM devices) don't have a standardized hardware interface like UEFI, you need dedicated images for each device, each combination of parts, and so on. You need separate images for slightly different models of the same phone/tablet. Otherwise the different components will not be detected by the OS.

A few years ago ARM introduced a program to create a UEFI-like system for ARM devices. Progress has been painfully slow, at least in the consumer space. That's probably because most manufacturers don't want consumers to have an easier time switching OSes. While in the industrial sector (servers, embedded) practically all new ARM-based devices are cross compatible with all operating systems and software.

https://www.arm.com/architecture/system-architectures/systemready-compliance-program

This isn't an issue with ARM itself, but a standard procedure for any new instruction set architecture. We've been through all this with x86 already. Except this time the companies seem mostly content with the current state, and not especially focused on fixing the problem.