Is it a fancy-pants upgrade of the base game? Is it a GOTY-like collection with the base game + Phantom Liberty + some additional content? I really love it when stores just put some vague bs in the description and you have to figure it out for yourself, because you're somehow supposed to know this stuff by default.

No additional content. The "ultimate edition" is just the base game + Phantom Liberty.

If you already own both Cyberpunk 2077 and the expansion, the ultimate edition adds nothing.
https://www.gog.com/forum/cyberpunk_2077/how_to_upgrade_cyberpunk_2077_to_ultimate_edition

I have both, but the launcher is still telling me to buy it. This is just like when I had my Xbox 360. I would boot up the console...and the first thing it would show me is and advertisement for the console I'm playing on. In both scenarios I'm being advertised something I already own. How bad does someone's amnesia have to be to boot it up, see the ad, and say "holy sh-t! I better go out and buy that right now!"?

I think this isn't some trick to buy multiple copies of the same game because the GoG checkout page will warn you about already having the extra content, and saying you can only purchase it as a gift.

Which launcher? GoG Galaxy? That is just a piece of useless software you do not need.

I'm running the game directly from the main .exe, and I have never been told to buy Phantom Liberty
...Apart from when I updated to 2.0, after having bought the expansion, but had forgotten to actually download and install it. *cough, cough* But that one is on me.

Not Galaxy, there is a separate launcher for the game.

https://imgur.com/a/tIr3hRT

It's strange you are not getting the launcher. I went to the game folder, double clicked "launch Cyberpunk 2077" and the launcher still popped up without Galaxy.

That is probably the Red launcher I've heard about. No, I don't have that one either.

I run Cyberpunk2077.exe, located in:
[X]:\GoG\Cyberpunk 2077\bin\x64

...and I just get straight to Cyberpunk 2077's main menu.