Plasma is the desktop environment developed by KDE. It’s for example desktop environment used by SteamOS 3.
As to how you learn this stuff: looking things up on the search engine of your choice helps. Not trying to be rude here, but you could have found all of that out yourself by searching for “plasma Linux”: lmddgtfy.net/?q=plasma linux
Also the arch wiki. There you can find all the info you could ever ask for.
" just use Linux" is a great piece of advice for most people because most people don’t care about the OS they use. They just use it. And they shouldn’t need to take a course to do so. Of course you are missing some things with this.
If you want more than you will need to go out and actively look for it