Mastadon is like a galaxy of federated planets that can speak/travel to Lemmy planets when they want to, but lemmy can’t really understand anything they say to each other and can’t really travel to their planets (mastadon is ‘people’ based like twitter, whereas lemmy is ‘subject’ based like reddit)

I dont know how kbin fits into this analogy. I think it basically is the same as lemmy and can talk/travel to lemmy but also only understands some mastadon