Most are pretty much actually k8s, though they tend to have different ways of handling the masters, my understanding is that it's the same k8s binaries and code
What I've seen is the k8s APIs working their way into other systems like cloud functions and servers, so you can use the same Yaml across vendors and products. This is further solidifying k8s APIs as an industry standard
Searching "managed kubernetes providers" is a good starting point to learn about the various offerings
