Bluesky grew out of a project at twitter whose goal was to create a federated protocol. Then when they were split off from twitter when Musk took over they had to start their own twitter-style platform to be the first player in that federated protocol. Now that that part is running they’re gradually building out the originally planned system of allowing more servers to be part of the federation: https://docs.bsky.app/docs/advanced-guides/federation-architecture