So, should there be apps that use it to transfer files between iOS, Android, Windows, and Mac without requiring them to be on the same network?

Google's QuickShare contains a reverse-engineered AWDL implementation that works on Pixel and a few other phones.

As for WiFi NAN: support for it seems rather limited outside of iOS and Android. From what I can tell the feature is barely tested on Linux and I can't find any generic Windows APIs for it either.

No WiFi cards for pcs support it.

Researched this for a bit: there is some hardware for PCI supporting it, but Windows 10/11 not, and Linux is still work in progress, so no real support on OS level, only for some iOS/Android devices.

Afaik the hardware supports it for a while now, but there is no firmware to expose the functionality.

That's not quite accurate, I don't think.

I've definitely used STA and AP modes concurrently on my Windows laptop with the operating system's built-in internet connection sharing function to help troubleshoot a problem in the field.

That was around a decade ago. It didn't take any extra effort on my part; I just told it to do the thing, and then it did that thing.

it might be interesting to use unused or extra wifi cards to support this. My pc motherboard has both wifi and ethernet and I only use the ethernet. That card does absolutely nothing at all.