“Play next” means it will add a selected item from library to queue right after currently playing item, keeping current song playing, and so next will be played song which was added by “Play next”.
Actually there are similar “Add to queue” and “Clear and play”, but still different, because in this case currently playing song will not be stopped and it will save a lot of time by manual removing queue items.
Under the hood it should do:
- Get current playing queue item position
- Remove all items after current item (optional)
- Add selected items from library into queue after current item
Play song, whole album is added into queue
Find other song/album and click “Play next”
Once current song ends, a new selected song/album will start playing