I have trouble understanding what the main differences between Presenters and Viewmodels in the scope of Compose are. Looking at different explanations I have the feeling they are confused a lot. What is a good definition of both and how are they different?

There are no "official" definition from Compose. Compose itself doesn't need them, they are a pattern for code organization. It's an old pattern (not saying it's bad!) so it has a lot of definitions, you won't find a canonical one.