General questions about computer science? About full stack/ App development/ etc

So I am currently a rising junior in computer science. However, I find a lot of the class material very restricted. Although very informative, i want to get out of the confined shell.

My question is, how do you decide which languages/ software combination you need to construct an app or webpage?

For example, I'm aware of django and virtualenv, but do I even need those softwares to construct a webpage? Will companies be impressed I know how to use these softwares?

Also, while installing I am not sure if i understand why virtualenv is important.

Also, what is a good software, method to construct iOS apps? Swift?

Thanks, Zevbda