Termux. A command line Linux system running on top of unrooted Android.
It lets you interface with your phone’s functions (GPS, calls, etc.), and install packages to extend functionality.
Turned my phone into a mobile network troubeshooting device, and I can ssh into my server on the go.
With AnLinux you can install a full standard linux system in it, including a GUI, and connect to it with a VNC viewer. (AnLinux is just a helper script linking to some dude’s repo, so if you are at all security-minded, you can also bootstrap and install any Linux distro manually).
So you could have a Debian with Gnome desktop running on your unrooted phone.