Do you have interns at work this summer? They may find this helpful (created by a summer intern at edX a few years ago):

@nedbat i recently started new job. I feel like intern, but have to chase everyone to get better understanding of the very old and very complex system. Nobody knows anything, from what it seems some things works, but some are broken for months. Near zero docs, zero tests.

I recall myself, when new dev joined the team at previous job: I spent days explaining and showing how things work. And I always felt, that I missed something.