Just ran into the same issue when installing R-4.0.3.pkg on my MacBook (MacOS BigSur). Can open R.app to the clunky R GUI, but typing in 'R' in terminal doesn't work.

Turns out, an R executable lives here: /Library/Frameworks/R.framework/Versions/4.0/Resources/bin/R

So I added this alias to my newly created .zshrc script:

alias R '/Library/Frameworks/R.framework/Versions/4.0/Resources/bin/R'

Now when I type in R, it opens... I swear this all happened seamlessly in earlier versions.