One of the show's only fully canon-confirmed supernatural mechanics. The items Julie is holding pin her to a specific moment, and the town's storeroom is organized like a library where each item is a bookmark.
Julie didn't storywalk this episode. She and Sara spent it painting Sara's house. Julie and Randall also had a conversation about how Township concepts leak into the real world as stories, and how those stories could become escape tools if reverse-engineered.
Mechanic settled. Open: who built the storeroom-library and who curates it. Leading candidate: Eloise (ties to the eloise-alive theory). Post-S4E6 a new read is gaining: Randall and Marielle may be undisclosed storywalkers. All three music-box survivors show structural sensitivity to the town that Julie has.