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!