BTW if I need to suppress an activity if the hotspot is removed, how can I retrieve it ?

You can tell your activity action to load by nothing, by asset only, by asset and owner, or by owner only. Your asset should technically be as unique as possible. Example as follows for CB Gallery.

gallery.photos.PHOTO_ID

So if I load by asset I can edit that activity and unpublish it, update it, etc.. Additionally you can delete by the asset so I could delete that activity entry. Note the delete usage supports a wildcard as well so if you delete the asset "gallery.photos.%" it would delete all photo activity for everyone (unless it's told to load by asset AND owner).

Do you have a doc somewhere for the API ?

No, you ideally should never have to directly use CB Activity API.

For instance it would be great to use the module per asset for other components.

CB Activity comes with a content prepare plugin and a module. Both support custom asset with substitutions.

But in this case the asset Id has to be dynamic and the module cannot be used.

Asset in all usages supports substitutions. So a module for example can have activity unique to the module by its module id (e.g. byusing [module_id]). More custom substitutions can be added as needed. The content prepare plugin can be unique per article if it's from com_content using [article_id]. Both support [page_id] which makes the asset unique to the current Itemid.

Can you explain how to integrate ?

Best way for 3rd party extensions to integrate is to just fire a CB trigger then act on it using CB Auto Actions. This ensures no dependency to maintain the implementation as that falls on us maintaining CB Auto Actions.