They update the roms while keeping everything provided by Qualcomm the same
so basically the kernel is frozen even if the android version is updated
so basically the kernel is frozen even if the android version is updated
Anyone can make a diff between the upstream kernel and the Qualcomm kernel. Maintaining these changes into later versions of the kernel will be quite challenging, but the base is already there.
That said, phones also come with plenty of binary drivers and those cannot be ported. That's an important reason not to bother with later kernel versions in custom ROMs: after all of your hard work, the end result will be missing important features such as GPU acceleration.
Custom kernels also exist for amd64 devices, often including workarounds and patches that are not in mainline to improve performance or compatibility.
As a vendor, that requires practically zero extra effort.
https://wiki.postmarketos.org/wiki/Devices has a list of devices that run either mainline or almost-mainline Linux. Only the "downstream" devices require vendor Linux kernels. Of course, hardware support is partial for most of these devices because vendors haven't contributed proper upstreamable drivers and volunteers haven't had the time to write them yet, but it's not like every ARM device needs a special kernel fork, that's just something ARM vendors do out of laziness.