There is currently a bug in CRAN's R installation package that results in it not correctly installing symbolic links to R and Rscript for commandline use. I've just verified this by inspecting the postflight script in their 4.0.5 installation package. This only impacts MacOS system releases of 20 and above (you can check with uname -r).
I've included more info here, along with what the "correct" fix should be: manually creating symbolic links to /usr/local/bin that point to the R and Rscript binaries themselves. If this is the current challenge, then this would be a far better solution to creating aliases or manipulating PATH in various ways, since it what the installation package intended to do (and presumably will again soon).
In short, if this is the problem, then Ashkan Mirzaee's answer (https://stackoverflow.com/a/67202173/2093929) to create the symbolic links directly is correct.