The levels are generated, and they are still the same every time
Nothing here is hand-drawn. Each level is assembled by code from its world and level number, which normally means a fresh layout on every attempt. This one seeds the generator from those two numbers, so level 4 of the Desert is built identically whether it is your first attempt or your fortieth.
That combination is worth understanding because it decides how you should approach a hard section. The level is not going to reroll into something kinder, and it is not going to surprise you either. Every death hands you information you get to keep, and the arrangement of saws, the archer on the ledge and the gap you keep missing will all be exactly where they were.
It also means the difficulty is honest. When a level feels unfair, you can settle the question by running it again rather than wondering whether the generator dealt you a bad hand.