I am using the stream "Recent Activity" and the filter is set to include the asset "article.%". But I can't filter out some Joomla content categories, in fact I don't even know the name of the asset. There are categories that I don't want to show in "Recent Activity".

Article activity isn't logged by category. It logs activity for every article and its asset is always "article.ARTICLE_ID". I suggest turning off that activity and creating your own using CB Auto Actions if you need it to be more specific. Will review adding a category filter for article activity/notifications in a future release.

forge.joomlapolis.com/issues/9342

How do I disable the "Make album cover" feature everywhere.

There's no functionality to turn that off. You've albums enabled and the media is in an album so it's possible to assign it as the album cover.

How does creating albums in stream work? A user uploads several images at once to the stream and an album is created. Another user then has their owen album for the stream? Or are all photos uploaded to one album?

Stream gallery uploads are always uploaded to a users profile gallery. When it does this it places them in the Upload album to keep them organized. This is very much like Facebook when you snap a photo and it auto uploads it does so to an Uploads album.

It's possible to override what gallery is used for stream uploads by changing the gallery asset within a streams parameters, but that's a more advanced usage and don't recommend adjusting it without first understanding how assets work to organize content.

When a user shares an image in the "gallery tab" group. The right side of the slideshow will be displayed differently than when the image is shared in a stream. Note that the right column is not full width.

Non-interactable media has a condensed modal window. That's working as intended otherwise you have a giant white column on the right side of the modal window containing basically nothing but name and avatar.

I am using the stream "Recent Activity" and the filter is set to include the asset "article.%". But I can't filter out some Joomla content categories, in fact I don't even know the name of the asset. There are categories that I don't want to show in "Recent Activity".

Latest CB Activity build release now includes a parameter for article create activity and notifications to exclude categories by their id.

You don't define an asset to exclude categories or filter. You can only filter categories at time of activity or notification creation. This is done by adjusting the "Exclude Categories" parameter in CB Activity > Parameters > Activity. Once the activity has already been created your only option is to just delete it.