what did it install via cmd?

Helps if you actually mention what game it was so someone can check.

Oh, that could be a lot of things.
Could have been silent running a library installer, could have been updating the registry manually, or any number of things. You could extract the install script and have a look yourself.

Don't they teach long-distance mind reading in Australia?

Had the same happen recently.
While the game's installation process, the CMD window opened to install/update (?) something called "Image management tool".

The game, respectively its installer file are pretty old, so I simply assumed, my Win10 system had to enable some extra stuff, to make the old game run properly.

If you're referring to DISM, it could be evoking that to get Windows to install DirectPlay. You can do the same thing manually with:-

dism /Online /enable-feature /FeatureName:"DirectPlay" /NoRestart

Other command line tools GOG installers call include sdbinst (used to 'patch' Windows to disable ALT-TAB inside specific games). Several of them here, eg, Dark Fall : The Journal force disable ALT-TAB because ALT-Tabbing out either gets 'stuck' in Windows and be unable to return to the game / crashes the game / some other glitches, eg, Windows gets stuck in 640x480 256 color mode, etc.

Very well possible.
I simply translated what my (set to German) CMD window displayed: "Tool zur Imageverwaltung" (image management tool).

A toolbar for your webrowser with shortcuts to pornsites.

You're thinking of Finland, where personal space is "knife fighting distance".