plugins, themes on multi-site clients site only? I.E. Avada or other?

I'm trying to wrap my head and others involved in this new multisite venture around it a bit more with every question - thanks for the answers to questions like this:

So in multisite we have the main network site that themes and plugins get installed.

Can we have something similar to wordpress.com that we can open up plugin options to our clients?

Same question in regard to themes...

We have one main site, and I know for a fact one client has Avada theme with a variety of plugins they will need to move the an install on our networked multisite.

How?

And what about themes on envato market licensing? if we network enable a particular theme that has been paid for - how do we monitor who enables the them on their page or not?

thanks again for the direction to some information I'm sure I missed in my search.