It’s worth adding here that on the rare occasion that you only have a .rb file you can still install it using the built in Install Extension button, without needing to know where the plugin folder is.
Simply make the .rb into a .zip file, using winzip or similar, then change the file extension from .zip to .rbz
Once you have a .rbz use Install Extension button to install it as described in the video.
I actually had a harder time installing them then learning how to use them, and looked only for the auto-install versions. It is something you need to figure out as/if you move toward creating texture libraries, importing free texture samples, rendering tools and such. More of building your computer knowledge or skills that will improve your confidence, especially if you have looked at or want to attempt dynamic components.