took a deep dive into how CEO Steve Huffman went from being Reddit’s co-founder to its much-needed savior at a difficult moment—and how he then became the villain at the center of Reddit’s still-raging protests: https://slate.com/technology/2023/06/reddit-protests-steve-huffman-api-chaos.html
Lemmy is Reddit.
Mastadon is Twitter.
Kbin is a bit of both.
They federate. So imagine using your Reddit account to reply to a tweet. And vice versa.
They don’t share the same servers - they just communicate with each other. Kind of like how I can use Gmail to write to a friend that uses a Yahoo Mail.