I am also unsure how buildx works, however I do know that the official docker build-and-push action is not compatible with podman (despite the podman-docker pkg), solely because it uses buildx. As well, compared to buildah builds, building with buildx is much faster--at least with default flags for buildah-build-and-push vs docker-build-and-push. I can say that buildx has an additional cache that must be cleared overtime, which might be related to the build speed difference.

If buildx is build kit then there's a good amount of granular caching involved, as well as the ability to build in parallel.

I use it to build x86 images on my M1 MacBook