The servers are abstracted from the platforms.
The platforms interact via ActivityPub.
ActivityPub = Protocols/Standards behind Fediverse
Fediverse = All the federated stuff in *this* universse

Mastodon => Twitter
PeerTube => YouTube
Friendica => Facebook
Pixelfed => Intstagram
Pleroma => Lite weight Mastodon
Nextcloud => Dropbox
????????? => Tumblr
????????? => Pinterest

Networks are very small at the moment but who knows how it grows ;-} Could be its time.