It's the internals.

He is training up people to work on new features for existing databases, or build new ones.

Not application developers on how to use a database.

Knowing some of the internals can help application developers make better decisions when it comes to using databases though.

Here is the playlist: https://www.youtube.com/playlist?list=PLSE8ODhjZXjYMAgsGH-Gt...

You can tell from the topics, it's related to building databases, not using them.