Seems to vary depending on whether I’m attempting to listen to music on my device (“attempting to” is the right word as it takes 15 seconds to load each individual song as it goes through a playlist) or listen to a YouTube video. It can go further into the background if it’s music on my device, no further back than a big window drawn over the screen if it’s a podcastable video, and must be in the foreground for a video IIRC.