kubectl get pod my-pod -o yaml

but what endpoint does kubectl use to get it?

If you add -v 7 or -v 6 to the command, you get verbose logs that show you all the API requests

Example:

kubectl get pods -v 6
I0816 22:59:03.047132   11794 loader.go:372] Config loaded from file:  /Users/jonas/.kube/config
I0816 22:59:03.060115   11794 round_trippers.go:454] GET https://127.0.0.1:52900/api/v1/namespaces/default/pods?limit=500 200 OK in 9 milliseconds

So you see that it does this API request:

/api/v1/namespaces/default/pods?limit=500

The API only returns the response in Json and the client can transform to Yaml when using -o yaml.