You link to communities like this: !wikipedia@lemmy.world
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”: https://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
There is another downside. The local and global feeds are potent discovery tools. But they only work if you group people with similar interests onto the same instance. Your proposal assumes a certain amount of homogeneity. If everyone is interested in the same content anyway then yes you can distribute it randomly. But all the people interested in Linux memes are already here. If we are to expand our reach we need to have instances catering to other interests.
And it also doesn’t work with international communities. German speakers for example go to feddit.org, precisely because that’s where German content is going to be amplified via the local feed and therefore easier to discover (for people an that particular instance)