ubuntu-for-termux guide
Automated Ubuntu with XFCE or LXDE on Android via proot. Repo ↗
Install
No root needed. Install Termux from F-Droid, never the stale Play Store build, then run the interactive installer and pick a preset or custom package set with XFCE4 or LXDE.
Connect
Start the VNC server with the command printed at the end of setup, open a VNC viewer, and connect to 127.0.0.1:5901 with the default password ubuntu.
Change that password on first login if the phone ever leaves your hands.