Controller on built-in types

I found this example for making a controller for ReplicaSets:

I thought that defining controllers for built-in objects was either not possible or an antipattern. What happens if my controller updates the Status for a ReplicaSet? Does the k8s control plane overwrite whatever status my controller sets? Would both controllers respond to new ReplicaSets and fight with each other?

I’ve read a bit about Server-side Apply (and managed fields). Is that something that will allow multiple controllers to own an object and that’s what makes this possible?