Sounds like you got it! Yes, plugins work just like how they work on VCV Rack or on many other programs: they are extra modules that you optionally install. So just like if you download someone’s patch they made in VCV Rack, but they use a bunch of modules that you don’t have installed, you will need to install them (from the VCV Library, usually) in order to play the patch.
Explained differently: the MetaModule comes with 160+ modules built-in. These are the modules from 4ms, Befaco, HetrickCV, Eightfold, Nonlinearcircuits, Cloned Instruments, and Valley. Then, if you want to get more modules, you can install a plugin. This is great because to get more modules you don’t have to install a whole new firmware, you can just load a small file that gives you new modules and you can even pick and choose what modules you want installed.
If you go to the main menu and click the + icon, you can browse the list of modules installed, which is all the built-in modules plus plugin-in modules.
Not all patches require plugins, it just depends on what modules the person who made the patch is using. But, from what I’ve seen, almost every patch posted here uses at least one module from a plugin.
At this point, there are (only?) 14 plugins, so it’s easy enough to just keep them all installed all the time and never have to think about it. The way to keep them installed is to put them on the SD Card like you’ve done, and then in the plugin tab in Settings, click each one that’s loaded and set Autoload on. Like this:
Then just make sure the SD Card is inserted when you power on. The downside to having plugins auto-load is that the startup time takes a little longer. If this is a concern then you can turn some of the plugins to not Autoload anymore, and just load them manually if you want to use them.