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! 

 Briar also supports an offline mode export to microsd card for dead drops 

 they also have the blog feature, where another person can pass around your messages  

 Moreover u can use briar with an old spare device as an mailbox from an place with perm. internet 
https://briarproject.org/download-briar-mailbox/ 

 Keet? 

 Yes Keet is like Briar P2P.  Unlike Briar though, can't do Tor.
But good for video 

 ty 

 I like the write up. I didn't know Signal was hosted on Amazon 

 SimpleX:

You pick your receive, they pick your send.