Kubebuilder does not exist as an example to copy-paste, but instead provides powerful libraries and tools to simplify building and publishing Kubernetes APIs from scratch. It provides a plugin architecture allowing users to take advantage of optional helpers and features.
Kubebuilder is developed on top of the controller-runtime and controller-tools libraries.
Kubebuilder is extensible and can be used as a library in other projects. Operator-SDK is a good example of a project that uses Kubebuilder as a library.
Sample controller: https://github.com/kubernetes/sample-controller
Book: https://book.kubebuilder.io/
https://github.com/kubernetes-sigs/kubebuilder
https://github.com/operator-framework/operator-sdk
Controller framework: https://github.com/acorn-io/baaah
https://eltonminetto.dev/en/post/2023-09-08-k8s-operator-sdk/