It works like this: Ditty.it allows anyone to create a 15-20 second clip set to a generic backing track. You have a set number of characters you can type, and then an automated voice will sing along to music using your lyrics.
There's an option to purchase actual songs (currently there are tracks by Sia and Future) but most people seem to be content with the free, ditty-created beats.
The Ditty app has been around since late 2015.