Depends on the stack you are using. I maintain an App List [0] that lists existing apps, which might help with getting an idea of the dominant frameworks. That said, popular technologies like Flutter are also coming to Linux Phones.

[0] https://LINMOBapps.frama.io

I just run X11 on mine so all the normal open source apps from my desktop work. Most of the mobile DEs don't seem to meed anything special other than your app handling a small screen well.

Depends on the operating system. For example on those with Phosh: https://developer.puri.sm/Librem5/Apps/index.html.

It's easy to get started, we started to write some documentation for Kirigami/Plasma Mobile: https://develop.kde.org/docs/kirigami/

It looks exactly the same as on the desktop.