All the plug-ins you see listed in the Smart Controls panel's Plug-in lists are Audio Unit [AU] plug-ins-- that's just the plug-in format that GarageBand uses [some other DAWs might use other plug-in formats, but GB only uses Audio Units]. The plug-ins you see in those plug-in menus are already installed -- they come with GB. You could buy additional third-party AU plug-ins, but until you've mastered the use of the built-in ones, you wouldn't be able to make informed choices.
As to how you could learn about plug-ins.. Well, plug-in processing is a big part of how a mix is put together, and people study that and practice mixing with plug-in processing for years to learn about it. Ultimately the only way to really master using the various kinds of processors is to use them in context of a mix, and practice with many, many songs/mixes.
That said, the course linked below discusses the various types of processors used in mixing. However that course is not conducted in GarageBand, and the plug-ins shown will usually be more advanced versions of the ones you might see in GB -- it's about plug-ins and processing in general, not specifically the ones in GB. But it does discuss most of the kinds of processes used in mixing. When you're ready to start mixing a full song with a lot of instrument/vocal tracks, you might find it a useful introduction to those tools as you begin to try to apply them in a real mix..