Regarding "101," 102," etc.

It is common, but by no means universal that U. S. colleges use this means of numbering courses.  Generally, 100 refers to Freshman (first year) courses; 200 to Sophomore (second-year) level courses, up to 400 and above for advanced or graduate-level courses.   Generally, the more advanced the course, the more narrow and in-depth its focus.

"101" is usually a basic intro course, "102" the  next intro course, and so on.  A first semester U. S. History course would be History 101 (say, colonization to Civil War); the following second semester course would be History 102 (say, Civil War to Present).  (As an aside, judging by what's happening domestically in my country, I have concluded that those courses are no longer taught, but that's another matter).   History 412, just to pick a random topic, might be an exhaustive dive into the Early Federal period (roughly 1790-1832).

Again, this is not a universal system, jut a very common, perhaps the most common system.

I do not know the origins of this system.

Here's a more detailed article from Cal State--Northridge:  https://catalog.csun.edu/policies/course-numbering-system/