A functional desktop Linux is hard. Getting desktop Linux to boot and run stuff isn't that hard in itself.

The problem is mostly drivers. They're made for Android specifically, and often for that device specifically as well, so getting them working outside of Android is hard. The second problem is of course manufacturer obstacles, they really don't want you to do that.

Technically getting a kernel and a working framebuffer is fairly "easy", because it's mostly already there, you could just replace the initramfs and run whatever init and software you want. It's getting the GPU to do stuff that's a lot harder. WiFi is alright but cellular is a complete nightmare. A lot of those are Java native libraries, which makes it non-trivial to use outside of the Android Framework. But all the kernel stuff, you already have ready to steal right from the manufacturer, or you can take the ones LineageOS uses. It's only a matter of getting a useful userspace.

And the phone landscape on Linux isn't that interesting, so people's attention have been around improving Android itself as it's much more capable and mature, and is open-source. If Android was closed source we'd have Linux phones already, but for many FOSS entheusiasts, Android is fine and much better polished.

If you're lucky, PostmarketOS might support your device well. If you're less lucky you might get a kernel that boots but you can only get a serial shell to it over USB. If you're unlucky, nothing exists, and you have to do it yourself.