Ok thanks. Still not as trivially easy as it should be then :(

No, it's very easy: https://existential.audio/blackhole/

Blackhole is Free and Open Source.

Also, Rogue Amoeba has a product called "Loopback". It's not cheap, but it's another alternative: https://rogueamoeba.com/loopback/

I tried Loopback as a paid user, and it wasn't nearly as stable/reliable as you would reasonably hope if you're an ex-Windows user converting to MacOS.

It still boggles my mind that there is no "Stereo mix" built-in.

Yeah I’m aware of Blackhole but honestly these sorts of hacks are fine for me a software engineer but not for regular users which is what trivially easy means to me.

There is no good reason it’s not possible in Quicktime to record the system audio along with the screen.