Ah, the pass-through from PDS to AppView? It doesn't store anything on the PDS, but you're right, PDSes that do implement that will have content pass through them in memory. Not necessarily all PDSes will/do implement that though. I think the long term expectation is that clients read directly from AppViews.

(As background, I've implemented both a full ATProto PDS and a full ActivityPub server, along with full IndieWeb and Nostr servers and clients.)