TeX uses -- (double hyphen) for producing an en dash and --- (triple hyphen) for an em dash, and that's pretty darn longstanding and well-established. And FWIW, the English writing conventions that I learned use em dashes as punctuation without an adjoining space—like this—but allow en dashes surrounded by spaces as an alternative – like this – so I frequently see spaced double hyphen used as the ASCII equivalent of dash punctuation and interpret it as the latter. I've never personally heard of double hyphen for em specifically, only either as en specifically or as a sort of ambiguous whatever-dash.

Then how do you type in an en dash instead? Also, according to LLMs, the en dash is used in far more languages than the em dash. Many languages don't use the em dash at all. Using the em dash for sentence interruptions seems to be specifically an US American English tradition: Most languages which use dashes for sentence interruptions use spaced en dashes instead.

because the double hyphen is a convention or placeholder for an emdash

A /more recent/ convention.

My memory of early Associated Press computer systems is that two dashes is the endash (or "nutt") and three dashes is the emdash ("mutt").