Okay, see, that's context even I forget, but you're right and bears repeating:

This `-` is a hyphen, which I love, even if I'm fairly sure I'm not using it correctly in grammar a lot of the time.

This `--` is an EM-Dash, apparently, which is also what I never use but I also thought was just a hyphen in a different context (incorrect!).

No, there are actually four different punctuation marks, all which look remarkably similar to the untrained eye.

1. We have the hyphen, which is most commonly used to create multi-part words, such as one-and-one-thousand.

2. We have the EN-DASH, which is most commonly used to denote spans of ranges. As an example, Barack Obama was President 2009–2017.

3. Then we have the recently maligned EM-DASH, which can be used in place of a variety of other punctuation marks, such as commas, colons, and parentheses. Very frequently, AI will use the em-dash as a way to separate two clauses and provide forward motion. AI uses it for the same reason that writers do: the em-dash is just a nicer punctuation mark compared to the colon.

4. Lastly, we have the minus sign, which is slightly different than the hyphen, though on most keyboards they're combined into the hyphen-minus.

By the by, they're called the em-dash and the en-dash because they match the length of an uppercase M or N, respectively.

It is probably even a hyphen-minus, so called because on most early keyboards one character had to do to represent both a hyphen and a minus. In Unicode, there is a separate code point for an unambiguous hyphen. There is also a non-breaking hyphen as well as the various dashes discussed here.

And "--" is absolutely just two hyphen-minuses, not an em-dash (—).