I’m not an ATProto expert either, but broadly speaking, it’s more of an aspirationally decentralized service, a lot of it is still centrally run by Blue Sky. Rather than split all responsibilities equally among hundreds of equal peers like ActivityPub, they’ve broken up the responsibilities themselves into modular parts: one thing stores user data, one thing is a firehose of everything happening everywhere, one thing is providing content moderation, etc., and users can kind of assemble their social network from all the parts they like. Many parts of it were designed with the assumption that the whole thing would have to operate at Twitter scale (that was the original idea, to make something that Twitter could move to) so it’s basically impossible for others to run some parts, like the firehose. The theory is that the real big infra stuff will all be so generic and neutral that there won’t even a reason for people to run their own anyway, and the more subjective or personal bits like personal data or moderation decisions will be easier to host alternatives for.