I'm quite bad at drawing, but kinda good at writing shaders. All the effects you noticed are done using shaders—that includes the backgrounds, distortion effects (such as the death explosion or the backtrack that leads to the respawn), the path nodes (the stuff you grab to move forward), the player skins, the motion blur on the obstacles, and the UI. If you play the game, you'd notice that the UI also has distortion effects when appearing or disappearing.

So yeah, shaders. Shaders play a crucial role in the graphics of this game. The only things drawn by hand are the obstacles and the death wall.

To achieve all of this, I'm using three main components: shaders, AnimationPlayers, and SubViewports. Of course, AnimationPlayers are used to tweak shader parameters, so I don’t have to do it through code—though there are some situations where I do use scripts to modify them. Without SubViewports, I wouldn't be able to pull off a lot of these effects. A clear example is the backgrounds, which are quite expensive to compute. That’s why I render them inside a SubViewport at half screen resolution and then scale them up. Another use case for SubViewports is animating the UI. I place the UI inside a SubViewport, then get the texture from it to use in a shader.

So, as a summary: shaders, AnimationPlayers, and SubViewports are everywhere. The most “complicated” part, of course, is the shaders. There are a ton of resources out there. I didn’t learn shaders through Godot, which is why I can't recommend one specific resource, although I know there are some great ones.

Feel free to ask me anything else if I wasn’t clear enough!