I can't understand if their "intro to database systems" is an introductory (undergrad) level course or some advanced course (as in, introduction to database (internals)).

Anyone willing to clarify this? I'm quite weak at database stuff, i'd love to find some undergrad-level proper course to learn and catch up.

It is an undergrad course, though it is cross-listed for masters students as well. At CMU, the prerequisites chain looks like this: 15-122 (intro imperative programming, zero background assumed, taken by first semester CS undergrads) -> 15-213 (intro systems programming, typically taken by the end of the second year) -> 15-445 (intro to database systems, typically taken in the third or fourth year). So in theory, it's about one year of material away from zero experience.

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.