Using the OS is allowed. As far as I know, one of the reasons why there are more Windows demos than for example Linux, is that Windows provides more graphics/sound APIs out-of-the-box while on Linux the OS itself does less you'd have to utilize libraries (which is kind of cheating when doing a 4k demo).

I think it could be summarized as "write a demo for the target platform as efficiently as possible". Doesn't necessarily need to be bare metal, there are JS demos that run in the browser as well. In that case the browser (with all its APIs) is the target platform.

The only way to achieve those kind of graphics in 4k is leveraging the Windows platform api. That is, unless you can drive a gpu and a soundcard from bare metal in 4k, while implementing the drivers and stuff for the hardware.

There's no such thing as bare metal today, unless you want to go from real to protected mode and fit a graphics driver in 4k. Basically impossible

Most 4K demos on Pouet the last years has been Windows only. Demoscene parties usually specifies exactly what computer they will run the demos on in advance so groups can optimize for that setup. The code sometimes only run on that particular setup.

On Windows, to show graphics, your program needs a window. To show a window you need WIN32 API.