What you are looking is called the cluster-driver. (See link below) TLDR; Rancher supports externally managed cluster IE you build cluster and take the Kubernetes components outside of Rancher (etcd, kube-api server, etc) then you import the cluster into Rancher while provides you the dashboard, fleet controllers, authentication.
Now Rancher deployed hosted Kubernetes distributions like EKS, GKE, AKS, etc because their is cluster-drivers for them and Rancher is basically just called the cloud provider’s API and requesting a cluster just like you would do in their web console.
As far as Rancher deploying other Kubernetes distributions on generic servers using the Rancher agent. That is limited to only RKE1/2 and k3s because those are Rancher’s distributions. If you want to deploy something else like OpenShift on your own servers. That is not something Rancher can do.
Note: There is some public discussions around cluster-api that might change this statement in the future. https://twitter.com/ibuildthecloud/status/1221101286210170885?lang=en