what do you mean by ‘how do they work’?
generally, VCV Rack modules needed to be ported/converted to work on the MM.
4ms have provided an api, which does most of the heavy lifting of this work,
but there are some differences/limitations (due to hardware etc)
so, depending on the modules code, this may be easier or harder for a developer to do.
how do you know which VCV modules are available for the MM.
check : https://metamodule.info
in particular : MetaModule | ModuleFinder
this is useful, not only because you can see what modules are available (and filter to find things) but also the cpu used… as the MM has much less processing capacity compared to you pc.
if you are just using VCV for MM patching, then you could just subscribe to modules that are available on the 4ms MM.
there are some threads on this forum about how you can do this ‘automatically’, but I dont bother with this.
personally, Ive found with VCV in general (and this extends to MM), with so many modules available, having too many listed in the module browser leads to ‘choice paralysis’.
so, I actually like to manually curate what modules Im subscribed to.
so my process is…
if I need something I dont already have, then I might use module finder to look for some applicable modules - I’ll then subscribe to a few, test them out - and unsubscribe to the ones Im not using.
similarly, if I see a new module announced on this forum that may be interesting, I’l subscribe to it, test it out… and decide if I want to keep it or not.
this way. I get to know a few modules very well, rather than just have a sea of modules that I hardly ever use, or know… sometimes less is more.
it also feels a bit more like how I use eurorack hardware, Ive only a ‘few’ modules available - f I buy a new module then I get to know it well, and sell it if I dont use it 
that said, we all have our own workflows / preferences. others, Im sure, like to have everything available at their fingertips. so you need to find a way that works for you.