> If someone here has gone to college, then they'll know what 101 means - course numbering.
At most colleges whose course numbering system I've seen, the kind of introductory course that the idiom "101" refers to would actually be "1"; at many of them, "101" would be an upper division class. Its an idiom that may have connected to some colleges' numbering system at some time, but it mostly exists independently now, and actually going to college doesn't actually make its intended meaning any more obvious.
I went to 4 colleges, applied to a number of others (not all US based), taught at a few and am giving advice to friends kids now going to college. With minor variations (3-digit v 4-digit course numbers) I've never, ever seen a college course with a single digit course number. Or 2 digit. But then, I've never met an English speaker so culturally tone deaf that they would question the idiom "Foo 101" as being anything other than "intro to Foo".
At most colleges whose course numbering system I've seen, the kind of introductory course that the idiom "101" refers to would actually be "1"; at many of them, "101" would be an upper division class. Its an idiom that may have connected to some colleges' numbering system at some time, but it mostly exists independently now, and actually going to college doesn't actually make its intended meaning any more obvious.