How do I actually install ROOT?

[Wile_E_Coyote]

You can easily get the list of available binary releases for e.g. “Ubuntu 20”:

curl -s -l "https://root.cern/download/?C=M;O=A" | grep -i "ubuntu*20" | sed -e '{s/.*\(root.*gz\).*/\1/}'

BTW. Immediately after:

@eguiraud

download a root.deb package and apt install root.deb

do not forget to execute: sudo apt-get purge root*