I agree with you, because I feel like I only got that one because I happened to get curious about CMOS (PMOS + NMOS) logic earlier this year, and remembered the general idea from before. Otherwise, I don't think I would have figured that out either. Google image search for CMOS NAND basically shows the solution, but the game doesn't tell you that's what it is until after you beat the level. I think seeing the answer, then immediate trying to reproduce it from memory is a good way to learn. Then if you try again the next day/week/month and are still able to remember it, then you've learned it.
I also looked up a solution for the full adder since I couldn't quite remember how it worked.
Tangentially, I've gone through similar material over time repeatedly in the games nandgame and Turing Complete, going through the Nand 2 Tetris course (on Coursera), building Ben Eater's breadboard 8-bit computer, reading "Code" by Charles Petzold and "The Pattern on the Stone" by Danny Hillis and "Digital Computer Electronics" by Malvino since that was what Ben Eater partly based his computer design on, and going over digital logic in CS-related EE courses up through how a CPU is made. But most of those barely cover anything below the logic gate level and I don't think any of them covered CMOS/NMOS/PMOS specifically which is why I got curious about them this year.
It's pretty fun though (my type of fun anyway), and I'm really curious to see how the rest of it goes since it's building a GPU instead of a CPU for a change.