What do you guys usually do when you have zero work assigned? Im on an internship in this company and I sometimes finish the work faster than they estimate so I find myself doing nothing for like 2-3 days. Any advice on what to do during this time?

Just because you have no work assigned does not necessarily mean you have nothing to do...
There is, more often than not, a bunch of minor stuff you can do on apps, even though you're already technically done with your tasks. See what you can automate(this is especially useful when developing apps for business), improve or optimize the UI, etc. In my experience , this will give you a great boost with your superior, especially at an internship level. P.S. this does not mean you should go and change the entire app core just beause you're bored...that would be quite overkill, and probably would not end well