Simple HowTo for adding a graphical display to a base Ubuntu Server 20.04 system.
user@ubuntu:~$ sudo apt update && apt upgrade -y user@ubuntu:~$ sudo apt install lightdm tasksel user@ubuntu:~$ sudo tasksel install xubuntu-core user@ubuntu:~$ reboot
When the system has rebooted you will be able to log into the xubuntu desktop. The -core indicates just the core desktop environment rather than the numerous recommended / associated apps.
Other desktops are just as easy, want mate, then its ubuntu-mate-core. Lubuntu is lubuntu-core. You get the idea.