Mastodon API question and thought on mapping instance connections
I'm not very confident to answer, but the general idea is that each instance is a peer, that can request data (post, activities) from other peers (instances) and offer those data to other peers.
Technically you can't get the whole picture of the network. There might be several instances formed there own little network and isolated from the outer world. Just like how bittorrent works, but instances instead of bt clients.