I’ve been meaning to join bookwyrm, but can never decide what instance to join, so I keep putting it off.

The way I understand it is “all” is not curated in any way*, it’s simply all posts across all communities federated with your instance.

*Some instances might be defederated from your instance, meaning any posts in that instance would not end up in your “all” feed, so that may be the extent of the curation.

I could be wrong, i’m relatively new to the fediverse myself.