Spotify removed this feature (their fullscreen mode now shows the banner art for the artist).
The album art is the visual companion for an album, this web app replicates the old functionality. After signing into Spotify, it's just making API calls to see what is currently playing on your account, and show the correct information.
Spotify sometime in ~2021 made a change to the display of the full screen player. It now looks something like this:
There are several bones to pick here, the biggest being that the Album Art is the visual companion for the record. The postage stamp sized album art in the bottom left corner makes the banner art the main event, and less preferable to showing the Album Art.