Hi folks, I found https://docs.rke2.io/install/containerd_registry_configuration giving a few examples, but where can I find the reference documentation for /etc/rancher/rke2/registries.yaml ?

That's it right there. What else are you looking for?

A short description of the elements in the yaml file. endpoint seems to be a table. What happens, if I add several hosts here? How do I set up a proxy for all anonymous container registries without hard wiring their names in registries.yaml? How comes that "docker.io" seems to match "registry-1.docker.io"? Etc.

Endpoint is a list, yes. If you add multiple, they are tried in order. You cannot. That is how Docker decided to set things up. Since they were the first registry ever to exist, some things are odd about it.