DEB-based distros (Ubuntu, etc.)
The PMS package will automatically do all repository setup for you on Debian 10+/Ubuntu 20.04+ installations, including obtaining the package signing key.
To enable use of the repository edit the last line of the file /etc/apt/sources.list.d/plexmediaserver.list
Find the line
#deb https://downloads.plex.tv/repo/deb/ public main
To use the repository, simply change this line to remove the comment character # and save the file
deb https://downloads.plex.tv/repo/deb/ public main