How did they build the Operating System animations?

Welcome, I have a quick question about the animation of the operating system etc. Windows. (out of curiosity)

1. What programming language did the engineers use to build these animations?

2. How do they do it? As in web development, we can use css to achieve this but how about the animation in the operating system?

Examples: Windows drop-down menu (from opacity : 0 to 1)