which artifacts are downloaded
Anything that is visible in under browse/search has been downloaded at some point. If you select an artifact you can also see when it was last downloaded but that information isn’t available in the REST APIs.
which artifacts are uploaded
All artifacts in a hosted repository must have been uploaded. Viewing the details on those artifacts will display the uploader, the uploader’s IP Address, and when the file was uploaded. Again, that information isn’t available via the REST APIs.
how often and by which users
We don’t track that information for downloaded artifacts as it increases the storage cost of the application significantly. If you want this kind of information you should probably setup something to consume the access logs. Maybe put it in an ELK stack?