I just saw a 5b tas and in level like 7 and 31 they could just clip through solid walls and not blocks. How is this achieved?

Good question! It is a technique found by Coppersalts, where if you align yourself perfectly on the left side of a ledge, you will fall off, but the game will think you are inside of the wall.

It is SUPER precise; if you're familiar with subpixel (distances smaller than a pixel) positioning in games such as Super Mario Bros. 1, well, that's exactly what's going on here, too. Because it relies on subpixel positioning, it won't always work even if you're in the right spot every time.

Basically, the way to get it is to just run off of a ledge a bunch of times until you get it. As far as I'm aware, there exists a setup for the trick only in level 15.