Should I buy a crank, derailleur, cassette, etc. all from the same company or are the parts of the drivetrain not super specific
It's complicated. 

There are many groups of parts that will work together just fine, and many that will not. You can mix and match brands in any number of ways and have it work fine.

Buying everything together is an expensive way to guarantee that everything will Just Work. 

You can put something together for much, much less money using random bin parts from your co-op. But, you will still have to pay for it with your own research work, in learning all the different ways that the components have to fit together.

The good news is, putting a bike together isn't rocket science. There is detail that you will need to learn, but you can learn it.