What is the best way to know the structure of a database
You can use "database Diagram" and add all tables to get a physical representation of the tables with PK and FK and some link between table but this is certainly not as detailed as you would like it to be.

If you don't have any dev documentation for those DB, that's still bether than nothing and with some manual work to check those link between tables, you can get something that is "workable".

Thank you