Install Termux and VNC Viewer. Open Termux and run these commands:
apt update
apt upgrade
pkg install root-repo x11-repo
pkg install proot proot-distro
termux-setup-storage
proot-disto install ubuntu
proot-distro login ubuntu
In Ubuntu shell:
apt update
apt upgrade
apt install sudo nano
adduser name #replace name with any username
exit
In Termux:
proot-distro login --user name ubuntu
In Ubuntu:
sudo apt install xfce4 xfce4-goodies
sudo apt install tigervnc-standalone-server
vncserver
vncserver -kill :1
nano .vnc/xstartup
Edit xstartup file:
#!/bin/sh
startxfce4
Press Ctrl+O and Enter, then Ctrl+X.
chmod +777 .vnc/xstartup
vncserver -geometry 1280x720 #replace 1280x720 with your resolution
Don't close Termux, open VNC Viewer, tap +, set address localhost:1. Save connection and enter it. Now you have Ubuntu desktop.