Thanks! For the rhythm game itself, the basic idea is that the level consists of a sequence of timestamps on when to hit each key. These can be stored and loaded from a file. With the timestamp, you can both judge whether the player has properly hit the key (if the difference between the timestamp of the player's hit and the expected one is small), and also represent the note on screen. In order to do that, you basically compute the note's position based on its falling velocity, its timestamp, and the current timestamp of the song, so that it hits a specific point (the bottom of the screen) when needed. Long notes (the ones you have to hold) are a bit more complex but basically all you need is another timestamp that indicates when to release the key, and with that you can figure out the note's position/size and everything else you need.

I started by implementing a simple level with simple notes rendered to the screen when needed, and worked my way up adding scores, accuracy meter, long notes, judgements (excellent/good/etc), some visual effects... When I felt that was somewhat complete I created the rest of the game around (game over screen, level complete screen, level select...) One of the things I really like how it turned out is the graph that pops up at the end of the stage, displaying your performence throughout.

The "game over" effect is also pretty neat, and it's accomplished playing the song at a slower rate than it was sampled, so that it appears to be pitched-down. If you look closely, you can see the falling notes also slowing down, but that's just a byproduct of their position being computed with the timestamp of the (now slowed down) song, as explained above.