The second paragraph isn’t accurate - Android uses the Linux kernel. Hardware manufacturers typically ship their own proprietary modules, but in principle there’s nothing stopping you from running a mainline kernel image as long as support for the specific device or SoC is present. Granted, this isn’t the case for the majority of phones, but this isn’t a limitation of Android itself and you’d run into the same issue trying to run postmarketOS or really anything else on an unsupported device.
I also want to point out that there really aren’t any good alternatives to Android (and its derivatives) at the moment. Mobile Linux distributions are getting better, but at least in my personal experience and from what I’ve heard from others it’s still a pretty janky experience in comparison and arguably is only suitable for enthusiasts.
The second paragraph isn’t accurate - Android uses the Linux kernel. Hardware manufacturers typically ship their own proprietary modules, but in principle there’s nothing stopping you from running a mainline kernel image as long as support for the specific device or SoC is present. Granted, this isn’t the case for the majority of phones, but this isn’t a limitation of Android itself and you’d run into the same issue trying to run postmarketOS or really anything else on an unsupported device.
I also want to point out that there really aren’t any good alternatives to Android (and its derivatives) at the moment. Mobile Linux distributions are getting better, but at least in my personal experience and from what I’ve heard from others it’s still a pretty janky experience in comparison and arguably is only suitable for enthusiasts.