Right now postmarketOS is a touch-friendly operating system based on Alpine Linux that runs on a handful of devices including the Samsung Galaxy Nexus, Google Nexus 4, 5, and 7 (2012), and several other Samsung, HTC, LG, Motorola, and Sony smartphones. There are also ports for some non-Android phones such as the Nokia N900 and work-in-progress builds for the BlackBerry Bolt Touch 9900 and Jolla Phone. Note that when I say the operating system runs on those devices, I basically mean it boots. Some phones only have network access via a USB cable, for instance. None of the devices can actually be used to make phone calls. But here's the cool thing: the developers are hoping to create a single kernel that works with all supported devices, which means that postmarketOS would work a lot like a desktop operating system, allowing you to install the same OS on any smartphone with the proper hardware.
One postmarketOS developer complains that Android's architecture "is based on forking (one might as well say copy-pasting) the entire code-base for each and every device and Android version. And then working on that independent, basically instantly incompatible version. Especially adding device-specific drivers plays an important role... Here is the solution: Bend an existing Linux distribution to run on smartphones. Apply all necessary changes as small patches and upstream them, where it makes sense."