yup we need more explanations like this. What we don't want is knowledge being privately held and people being told "oh dont worry it all works just accept it". That's a recipe for alienation and disaster.
I think there's a balancing act that we can achieve to this end. To use driving a car as an analogy, someone doesn't need to know 100% of how a car works in order to learn how to drive it. But there are some basic mechanical things they can learn about that makes it safer for them. Like, how to check their oil.
If the barrier for entry for any social media, let alone Federalized social media, was 100% knowledge of everything, then no one would use it. However, there are absolutely a few things that should be known to stay empowered and safe.
what is a "fediverse"
The Fediverse is a collection of severs that talk to each other that publish info and host files. These severs are owned by lots of different people, they are hosted in many different locations, they run on different protocols. Kbin runs off of the Activity Pub protocol, so it can talk to other severs using Activity Pub. (The majority of the Fediverse uses Activity Pub currently).
Kbin has a Magazine where you can discuss the broader Fediverse: https://kbin.social/m/fediverse
What is a Magazine
A Magazine is what Kbin calls communities. We are currently on /m/RedditMigration (/m/ standing for Magazine). I just mentioned /m/fediverse. Some other good Magazines to note are /m/help for asking questions about Kbin, and /m/kbinMeta for discussing more administrative related issues about kbin.
exactly where is the info stored in the world
Lots of different places. If I recall right, Kbin is hosted both in Germany and Poland. But Kbin is not the Fediverse. Kbin is just one little part of it. So far, I've found this resource to be good to see more info about different servers: https://fedidb.org/network.
exactly who has control of which info
That is an extremely broad question. There's lots of different information, different context in which that information is important, different touchpoints where different people might have control over what you see or don't see.
Magazines or communities have moderators, much like how Reddit had moderators in Subreddits. Sever administrators can determine what instances they want to federate or defederate with. Individual users can block instances or other users.
Long story short, what makes answering that question so difficult is that the Fediverse is very decentralized. If we were talking about Reddit, then ultimately the puck stops with greddy little piggy /u/Spez, he controls your information. But since we're on a much more decentralized platform, authority is held between many different people. I see it as a good thing imho.
Is there a diagrammatic wiki where all this is explained?
I have to answer a question with a question: what is a diagrammatic wiki? I've not heard of that before, and a quick google didn't really give me a clear answer.
As far as where to go to have all this explained, the wiki article on the Feidverse is a good start imho if you really want to dig in: https://en.wikipedia.org/wiki/Fediverse. Otherwise, the magazines I listed earlier are good resources. There will be more resources in the future, this is all still very very new, particuarly on kbin. I've been piecing this all together by using the platform and learning about it for about 2 weeks.