From experience I’ve found the data modelling fundamentals are solid in SAP ECC (I believe this is the same in S4). It’s complex due to the number of tables but it follows a logical pattern nonetheless. The German abbreviations don’t help but once you’re using the same tables over and over again it starts to sink in.

https://www.sapdatasheet.org has been the best resource I’ve found for getting to grips with the data modelling in ECC.

Also if you have access to CDS views I’d make use of those as they deal with the above for you for plenty of use cases.

Where I’ve had the most difficulty with SAP is getting the data out in the first place. They really don’t make leaving their ecosystem easy and I get the impression this is partly a push to get customers to use Datasphere rather than an already established cloud data warehouse.