Understanding Table design in database
The database is an encoding of the business rules. Without understanding the rules there is no way of making sense of the database. Take it piece-by-piece, focusing on one area until you have a reasonable understanding of it.

Even if documentation exists I like to draw ER diagrams from scratch as it helps me remember the connections. It also allows me to compare how I would have done it to how it is currently implemented. The differences are an opportunity for further learning.

Reading the code will bring out further "virtual" FKs and alternate keys that are not declared explicitly.