I was not really good at puzzles as a kid. As I got older though, they got easier. I spent a lot of time alone teaching myself how to really look at the pieces and think about how the shapes went together. Same thing for lego sets, I had to teach myself how to interpret the 2D directions into the 3d shape.
That said, one of the aspects of the brain is spatial reasoning, mine is pretty good naturally even if it took practice. Other people, just don't have it. This is a normal variation for humans and I don't think is particularly associated with ASD.
wiki:
https://en.wikipedia.org/wiki/Spatial_visualization_ability
Take a test (this one is really hard):
https://www.123test.com/spatial-reasoning-test/
Edit: Legos, especially the sets, can contain small pieces that can require fine motor skills to assemble. Even when I understood what to do, I sometimes had trouble with actual assembly because my fine motor skills are not great and take a lot of patience.