I remember running kali linux once on my phone with (termux+vnc) and a vnc viewer app watching some random youtube videos a few years back

So I feel like, Something like this was/is possible but its immensely hard for something like this being used especially when a desktop os on a phone is so bad ergonomically speaking unless you have a keyboard mouse connected

A better option iirc is to use something like kivy[0] directly with termux, not sure if java might have direct options too or not.

[0]: https://github.com/kivy/python-for-android