oracle – How to organise and understand a terribly modelled database

I have recently joined a company where there are hundreds of schemas and thousands of tables. All the schemas we have are from other vendors who have allowed us access to their database. The entire database is poorly documented (little to no documentation). Vendors also don’t care for documentation, hence they don’t provide us with any documentation.

The other employees themselves find majority of their time figuring out what the data represents and if the data is current. There are numerous front end tools (not built by our company) which allow us to pull data (which is correct and relevant). However, then the issue becomes that we don’t know the schemas, tables or columns the front end is using to pull the data.

I cannot create new tables as the tables are owned by 3rd parties who have allowed us access to their database. How can I go about documentation and how do I figure out what schemas, tables and columns front end tools are using to pull data.

Tables don’t even have foreign keys to figure out how they are linking to other tables. For example in tableA there may be a column called car_id, but in tableB there is a column called cid (which is the car_id). They both link together.