The ROOT team provides ready-to-use binary distributions for some systems (see “Download a pre-compiled binary distribution” therein), including various Ubuntu versions, which you just need to download and unpack.
For Ubuntu 20.04 / x86_64 / gcc 9.3: https://root.cern/download/?C=M;O=A;P=*[uU][bB][uU][nN][tT][uU]*20*