I'm apparently supposed to take over some projects from the guy who sits next to me, who's leaving the country indefinitely. I don't even understand what I'm working on myself.

There should have been knowledge transfer and documentation. Try for it asap.

Relatively new project under development. I'm supposed to unit test data services, and I know the theory of the architecture, but not how to manually run them.. There are a lot of diagrams in our knowledge base on data services, but no real documentation. Also the guys who wrote these particular services are either on parental leave or on vacation. I'll have to wait it out.

This happened to me at my last company, I was a junior and two seniors left, so I took over every project... fight through it