In Bluesky, a custom feed is a service with its own DID that answers with a skeleton of post URIs; the app's server hydrates them; the feed is declared by a record in its author's repository; any HTTPS server can host one.

Algorithmic choice is a protocol feature, not an app setting.

In Bluesky, a custom feed is a service with its own DID.

Ask it for a feed, and it answers with a skeleton: a list of post addresses, in order. No post content. Just which ones.

The app's server fills the skeleton in with the posts, the authors, the counts.

To publish a feed, you write one record in your own repository that points at the service. Anyone can find it, pin it, and share it.

The ranking is the feed's business. The data is everyone's. A feed can run from any server that answers HTTPS.

Check yourself. What does a feed generator return, and what does it not?