They run their own centralised relays.
Discussion
What we need is a local relay that stores events in the background so you can check it first for instant response
have you tried damus android or notedeck? Thats how it works. except it only reads from the local relay, remote notes are just fetched and dumped into it
I've got notedeck on my Mac and I was among your early beta testers for Damus on Android, but I didn't contribute much feedback, so guilt prevents me having a strong opinion on this 😂
Love notedeck BTW