Alright, I have a working example of use of scrobbling going here: https://gitlab.com/soapbox-pub/soapbox/-/merge_requests/2567
Basically if you scrobble to Pleroma’s API it will show your song as “currently playing” under your name (truncated) for a duration equal to the amount of time the song or video or whatever you scrobbled lasts.
If you don’t scrobble anything or haven’t scrobbled anything recently, it shows nothing. Two examples of posts, one with and one without the feature.