Containerd is the docker runtime, so it's still used to run/manage containers on the kubelet. Crio is another option: https://kubernetes.io/docs/setup/production-environment/cont... . I think the distinction in the docs there is that with just containerd, you don't get all the docker tooling

My understanding is that docker was the most common, but I'm not actually sure if that's still the case. CRI is the interface k8s uses to manage the container engine, and I think CRI-O is the most lightweight, but also gives you less/worse tooling for debugging if you need to drop into a kubelet.

The images are just OCI-spec now, but if memory serves, Docker used a different image format in the past, and it was another containerization company (coreos/rkt) working on an open standard which eventually become OCI

Yes, you use OCI images. The OCI format is a specification for container images based on the Docker Image Manifest Version 2, Schema 2 format. Now, there are multiple ways to build these images, some which are better than using Dockerfiles, though mostly people still use Dockerfiles.