You can create multi-platform images (actually manifests of single-platform images) without buildx, and buildx isn't enough to create multiplatform images. In its default configuration, buildx can usually build images for different processor architectures but requires CPU emulation to do it. If the Dockerfile compiles code, it runs a compiler under emulation instead of cross compiling. To do it without CPU emulation involves configuring builders running on the other platforms and connecting to them over the network.

I don't know if it supports building images for multiple operating systems, but it probably doesn't matter. I've only ever seen container images for Linux and Windows, and it's virtually impossible to write a single Dockerfile that works for both of those and produces a useful image. The multiplatform images that support Linux and Windows are probably always created using the manifest manipulation commands instead of buildx.