Hello Armin,
Hope you are well.
This can tend to be a very wide topic.
Normally huge databases are attached to an increased size on Audit, Attachments and Annotation tables.
Each table has a different path to understand how to reduce it.
Would you mind sharing your top 10 tables showing row counts, reserved size, data size and index size?
Using SSMS you can generate a report and then share only the top 10 tables by size.
Looking forward to your reply.
Thank you!