So far it seems to be Mastodon, with some tweaks, and the improvement that while it's federated in theory, right now there's only one server. This addresses the critical flaw people point out with Mastodon, which is that because it requires you to choose a server it's too confusing. The requirement to choose a hosting provider is why email famously never took off.

Considering there's no explication on bluesky's website, i don't understand why this comment is downvoted

Sunlight reaches Earth's atmosphere and is scattered in all directions by all the gases and particles in the air. Blue light is scattered more than the other colors because it travels as shorter, smaller waves.

This is why we see a blue sky most of the time.