As you probably know, tons of software can be installed in Ubuntu right from the command line via apt or from the Ubuntu Software application on Ubuntu's desktop. The way this works is that Ubuntu will query a repository which contains links to the various software you may wish to install. It searches for the package you specify, and then downloads and installs it.
This is a companion discussion topic for the original entry at https://linuxconfig.org/how-to-list-and-remove-ppa-repository-on-ubuntu-20-04-linux