I believe the drivers for pmos are all written from scratch, at least for the iOS devices. I’m sure some things could be borrowed from Lineage device trees. Their Wiki says that most of their supported iOS devices dont have anything past basic screen support, so I would guess that the shared base is only marginally helpful, unless alpine is no longer a part of iPhones (iPhone 6 is the oldest on the supported list I think?)

I’m going to say Alpine/postmarketos. The reason I say both is pmos uses Alpine as a base, but a lot of the code is built from the ground up as its a linux distro designed to run on mostly ARM devices (old phones and tablets. Even some old iOS devices!)