There is initial support for Android apps via Anbox: https://www.youtube.com/watch?v=VGpfleG8TMg

Also an Android ROM called GloDroid exists: https://peertube.co.uk/videos/watch/a368a023-ce44-44f0-a42d-...

Native app development depends on the OS, it differs whether you would develop for Ubuntu Touch, the GNOME on Mobile effort by Purism or for Plasma Mobile.