The main difference between arm Linux and arm android is how applications are packaged, (and android really wants you to build Java interfaces) but that’s easy enough to guy around. It takes some development, but doesn’t need to be a “new codebase”. The harder problems are making the ux work with different from factors.

Actually, Linux is a good example of how this works, since you can build Linux for dozens of different CPU architectures (x86, x86_64, arm, risc-v, powerpc, and the list goes on)

There are places in the code where there are bespoke instructions to optimize for a given architecture, but overall, it’s still one massive “codebase”