@jenniferplusplus @mikedev If someone sends you an activity, you can look up actor, attributedTo, inReplyTo and other referenced objects on their server because they should already have them in cache.
Once you have an actor object, you can start interacting with its clones listed in aliases property.
Publishers can also provide location hints in query parameters. Take a look at "Actor IDs" section: https://codeberg.org/fediverse/fep/src/branch/main/fep/ef61/fep-ef61.md#actor-ids