@Andreas Actually, it's none of the above, but if you wat it is more on the one-way side.
First important thing: Not whole instances federate (all their stuff) with each other, but just users/ groups/ communities/ magazines. All of these are "actors" and the https://activitypub.rocks/ protocol is designed about them exchanging information.
This exchange is mostly push-based along follow/ subscription relationships. This is what makes it mostly one-way.
Replies are delivered explicitly.
@Andreas Furthermore, fetching and pulling data back is also possible, but for scalability reasons only happens for minor specific cases.