I’ve used Termux with proot-distro to install Debian with xfce4 on my Android phone.
I can use it on a monitor with a usb-c to hdmi cable, or just vnc into it from a PC, or use the phone itself with RVNC to have a desktop “in hands”.
Sensor data is sadly not working properly, so no CPU usage and temperatures are a bit off as well.
How?
You can install Termux from the F-droid store, which is a Android terminal emulator and Linux environment. Then install the proot-distro package and you can install stuff like Ubuntu, Debian, Arch…
you dont even need a rooted device.
As OP said, you use Termux. You can use AnLinux or Andronix for easier setup with Termux. You will likely also need a VNC Viewer to actually see and use the desktop. It’s been a while, so I don’t recall exactly how to do it, sorry.