Start by understanding what business solution the database is providing. Are you looking at a way to track widgets being built? Healthcare? Financial transactions? Customer Service?

That will help you have some sense of how the data flows through the system. If you can have someone show you a front end, sit down with them for that. While they're showing you, write up statements you can reference later looking for referential integrity (customers have orders, create contact records, each contact is tracked but can be passed to multiple departments, etc.)

Then hope names make sense. Many times there are reference tables with the front end descriptions for things that are only referenced by key values in the tables. If you find one that you want to track, find every one of the columns named like that key, and/or find foreign key constraints.

Look at index usage, query store, any other tools within the database that are built to show you where the data is being used.
Views, stored procedures, reporting from the database are all good places to look to find out how it works.