The first subassembly is a gear.
The second subassembly is a spire made of smooth wheels.
This project keeps reminding me of a truism: "Experience is how you avoid mistakes. Mistakes are how you get experience."
In Session 1, I showed you how these parts all come as part of a lasercut board. In physics, there is a tradeoff between weight and strength. Many of these parts have thin areas that are relatively fragile.
I know this because I have made models in the past, and observed that thin parts are prone to breaking -- especially if you have to pry them out of surrounding material. So I understand the importance of carefully pressing around the rim of a part, rocking it gently back and forth to loosen the connections, NOT bending it, NOT using anything thin as a rocker or lever, etc. I know this from having broken other, similar things in the past. I could take one look at those parts while still in the board, count up all the thin sections, and rate this as a model for experts. That's experience, and it comes from mistakes. I can tell you about it, and maybe help you learn a little faster, but that doesn't give you a tactile feel for how hard you can press and where. In the end, you're not going to learn how to make fancy models without breaking a few parts along the way.
This is why making mistakes is a natural and necessary part of the learning process. If you're not making any mistakes, you're not learning, you're coasting.