Keyboard layout specific. Macs with their default English layout use “option-shift-dash” which is really easy to remember (and relatively discoverable, as such things go) which is why using proper m-dashes (not just double-dashes) used to be a strong indicator a poster was using a Mac, before LLMs took the character over.

On iOS you type it by pressing dash and holding until alternative options come up, same way you type e.g. accented characters.

In emacs, c-x 8 RET prompts you for unicode character names (or hex) so for rare use you can just spell it out. There's also C-x 8 _ m for em dash and C-x 8 _ n for en dash. (Hit c-x 8 c-h to get a full list of those bindings, like any normal secondary map - they're about as idiosyncratic as the XCompose bindings, but you might find some of them "stick" in your head better (I personally like "C-x 8 1 / 2" better than "Compose 1 2" even if it's a lot more typing...)

Macs have a native way to do dashes: option- hyphen for en-dash and option shift hyphen for em-dash. On Windows there are some application-specific ways that make sense, e.g. in Office, but outside that you’re on your own and have to use the “hold alt and type the character codes” method! Or charmap.

> How do you type an em-dash? Is it OS specific?

On Linux X11 at least, you can enable the Compose key and then press `<Compose>---` which results in — and `<Compose>--.` which gives you –