How to deal with complex historical DB model?


I am currently working on a project that have been developed for over a decade.
The question is general but is actually to be applied for MS SQL Server.

When it comes to C# code I rather feel fine. But for the SQL part I rather feel lost.
I sure need to learn quite a few things about T-SQL and SQL Server itself, but it is not really where I struggle. Learning new things on my own is quite a habit.

The model is tentacular, I am revoked the rights allowing to generated diagrams in SSMS. There is a document displaying the tables (kind of ERD but the French way). There are so many tables it is hard to read. I am reassured by a senior dev telling me he doesn’t get it. But he knows the database model very well. On my side though I have a hard time to get a grasp over the DB model.
For I am highly visual and I think faster that way.

It there any pro tip or suggestions you could share with me ?
How would you go at understanding a complex DB model ?

Kind Regards.

Hi I think if you understand the application, then you will understand the tables and db. So maybe try to do a user journey through the app and then try to understand what is the important data that might need to be stored in the tables at each step in the app. For example for an ecommerce site there might be some important data to go inside tables like orders, products, customers etc.

Also I think if you just understand a few important tables, then you can understand the entire db, because likely the other tables will be related to the important tables somehow. Just like Mosh says baby steps is the key, if you try to understand the entire db as a whole all at once its a recipe for disaster, but if you start by understanding one or two important tables, then start reasoning up from there, then in time you will understand the db