> spaces around em-dashes is a question of style, not pre- or pro-scribed by any specific typographical rule

Writing and publishing style guides like Hart's Rules (Oxford Style Guide) & Chicago manual of style have the 'em' dash use as a parenthetical closed or "no spaces" dash.

In British use – Hart's Rules – writers will choose the 'en' dash with spaces as a parenthetical dash, where US writers/publishers choose the closed 'em' dash for the same thing.

Imo, there is a conflation of 'en' dash and 'em' dash going around due to the ease of smart-dashes auto-correction turning (--) into 'em' dash with the 'en' dash and non-auto-correct 'em' dash needing a key-combo.

Common everyday typing online, I think people will simply use what is convenient and "good enough" -- a single hyphen dash as an 'en' dash or 2-hyphen dashes that may or may not auto correct into an 'em' dash. I prefer mixing spaces with a 2-hyphen dash 'em' dash, but I'm not a published writer so I enjoy doing wild things like that