A lot of that is polish to make the game feel great. You should play Matty’s game Jumper. The game was made with Game Maker back in the early 2000s, which was targeted to low-code users back then. The game mechanics are very similar to Celeste. Despite the lack of polish, it’s still a very entertaining game.

It's kind of weird how terrible this code is (according to software design 'best practices'). 5k lines in a single file, tons of repetition that could be replaced with more compact code etc.

I would think the author was a novice programmer, but looking at the creator's history they've already made Towerfall, another hugely acclaimed game (and one I still play to this day), about 5 years before this was released.

Yet the game itself is extremely polished.

I kind of have no idea where to take this thought, except that successful auters have very different sensibilities than garden variety programmers.

Frankly this still doesn’t seem technically challenging. It does seem like a different sort of design challenge though to get all of the little mechanics correct. This isn’t like inventing a new algorithm though. I suspect implementing this is more like constantly iterative tweaks and “feeling out” whether the controls are right.

The end result looks complicated but no sort of specialized engineering skill seems necessary to achieve the result. Just grit and design talent.