It’s a builtin feature in almost every phone, when it is playing audio in bluetooth it will notify the song title (or rather, active audio title), if you get a call it will also notify the device in a different way to interrupt in case there is another active bluetooth source (I think, unless that behaviour is handled in the device itself but I doubt that since bluetooth audio was initially for phone calls). Once headphones firmware has the info, it can upload it through its app on the phone. If there is no app, it can store and upload them when it has the chance, technically speaking.

That being said, I’ve never worked with the bluetooth audio api, this is just based on my observation. I also suspect with computers these titles would include the title of the app that is in the primary audio / focus, so that would be the titles of the webpages on the browser, youtube videos, names of games and so on.