What servers do messages get stored on?

Signal:
Amazon

Session:
Decentralized database w/ pay in crypto

XMPP:
Self-hosted federated

the word "federated" means cross-communication of servers like email

Matrix:
federated (in theory)

SimpleX:
two federated self-hosted servers,
you pick your send, they pick your recieve

Briar:
No servers, peer-to-peer
They gotta be online

Nostr
poster picks distributed relays

Ethereum push.org notifications
federated then fetched by wallet
(this is NOT on blockchain, it's like nostr but only 1 relay)

IPFS
Torrent CDN getting it from a federated pinner (host)
IPFS is really a CDN
CDN means content delivey network, or servers all over distributing faster

Akash & Flux
You're renting servers from their network for your software
The blockchain just keeps track of who owes you service

Add on to the list with other ideas!