It's not actually complicated, since at any particular school it's quite simple and consistent. It's just that each school can have its own "simple and consistent" method.
Probably a majority of US colleges use the "English 101, "Physics 101" etc. to describe a "first course" in a subject. This usage is common enough for everyone here to recognize it as a "start here" kind of slang.
My own university used an 8.01, 8.02 (physics); 2.01, 2.02 (mechanical engineering); 14.01, 14.02 (math); 6.01, 6.02 (electrical engineering) structure, but I've only seen (or only noticed) one other US university doing it that way.