Understanding source code

I was trying to understand the source code so that I can contribute, there is two codebase for apiserver one is in route kubernetes/pkg
/kubeapiserver, and other one is on kubernetes/staging/src/k8s.io/apiserver/
If apiserver code is maintained in seperate package, i don’t find kubernetes/pkg
/kubeapiserver code is referenced on stagging level package, my question is how code inside “kubernetes/pkg
/kubeapiserver” is used or make contribution to apiserver

If you want to look at APIserver, start with cmd/kube-apiserver