To answer your question about Android apps, there is an application called Waydroid that can run on Linux phones. This essentially emulates Android and you can install apps on there. Some Play Store apps require access to Google Play Services, and even though MicroG tries to emulate it without being as privacy invasive, it is not perfect and some apps won’t run well or even at all.
I only use it for a few things that do not have any way to access through a web browser.
As someone dailying a Linux phone, I will say that there were no issues getting it to work on T-Mobile. The only thing I did was manually enter the APN details, but that was an OS thing.
From what I have seen on forums Verizon won’t let you at all, and AT&T might take a bit of work to get them to whitelist the IMEI on their network.