OCI image manifests can specify platforms and architectures. From the end user’s point of view it can be all the same invocation.

Docker natively supports Windows, and it is low lift to make native Windows images for many common programming environments.

Does anyone use it? No not really. It makes a lot of sense if you need Windows stack stuff that is superior to Linux, like DirectX, but maybe not so much for regular applications.

There is also macOS containers, a project that has a decent proof of concept of a containerd fork that runs macOS container images. In principle there is a shorter path of work for so called host process containers, but fully isolated exists for macOS, it could work with e.g. Kubernetes, and people want it and it makes sense, and it sort of does exist.

The difference between cross-platform and “cross-platform” as you’re talking about it is really having some absolutely gigantic company, like Amazon or Google, literally top 10 in the world, putting this stuff into a social media zeitgeist.