It's GNU/Linux, you can build apps however you want. The PureOS distro is based on Gnome so GTK is the blessed framework for app development. However, Qt works just as well, and is used a lot in KDE's Plasma Mobile. And if that doesn't suit you either, any other framework will work, with the caveat that they might not support mobile form factor natively.

Yeah, I was hoping Qt would be a first-class citizen. My previous experiences with GTK on mobile have been bad (little or no docs, unconsistent APIs, etc etc...).

I had a look at their docs and I like the focus on Python, I'll probably give it a go. If I can get a decent result with PyQt I'll probably order one of these bad boys.

Be aware that Qt's widget library isn't exactly well-suited for mobile usage and mobile apps usually use QtQuick with frameworks like Kirigami.

PureOS Amber unfortunately has a too old Qt to be able to say that it's a "first-class citizen" there :( It's going to change with a coming version that's based on Debian Bullseye though. Meanwhile, Qt apps run very well in Flatpak on Amber already.

>Be aware that Qt's widget library isn't exactly well-suited for mobile usage

How so? It's my understanding that QT has been the industry standard for embedded systems.

If you make your own UI, yes. But if you make a desktop-style application that's supposed to adapt its UI to small touchscreens, you're going to have a hard time with Qt Widgets. Even KDE uses Qt Quick for its adaptive apps.

PinePhone with KDE Plasma Mobile is going to be sold in the next few weeks so you might be interested in that as well!

Qt fines fine on a Librem 5. I personally was able to run musescore on it.

It's just a Linux computer, so you can use whatever you like : Qt, GTK, web, Flutter, LVGL, ImGui, custom OpenGL UI (like Blender & Godot), etc.

But you might prefer a library that has mobile-ready widgets. For that GTK is actually not bad, especially with LibHandy, but QT/Flutter is also be great choice. It's up to you

You should see how it works on the alternatives: you have to ID yourself to a multinational just to download the compiler and toolchain!