However I want to know how to find federated instances from a certain instance instead of the sites if I can. Doesn’t ActivityPub have such a method itself?
It probably has a way, since kbin has a federation button at the top right in the sidebar. kbin.social’s bar has the “instances” list empty there. I have no kbin account so I cannot tell if it’s private and the list would be nonempty when someone logged in kbin.social views it. Haven’t seen a dedicated “page” to do that, as Lemmy does in the https://kbin.social/magazinesnces url. You can view all the magazines on the instances you are federating, as they appear in kbin.social/magazines -all the magazines with an @instance suffix are magazines that are currently federating-.
In my opinion, currently federating instances list may not be very useful for discovery and exploration as they tend to get very large and for exploring their magazines properly, they are already in the magazines list of your instance. Any instance not already federating with your instance may start federating -unless blocked- when you subscribe to some magazines, so for community/magazines exploration and discovery, probably a list like The Lemmy Explorer -which includes kbin instances now too- may be of more use. I understand this is not what you want but it is how federation works with apps following the ActivityPub protocol. At least the most popular ones i know.
Blocked/defederated instances lists are more likely to be useful in this sense because they usually are not very large, and they tell you what you have no chance of communicating from your instance.