Users currently tend to push events to massive relays but decentralization is still a goal:

https://raw.githack.com/dtonon/oracolo/master/examples/fiatj...

https://github.com/nostr-protocol/nips/blob/master/65.md

Still, the short answer to your question would be "yes."

Not necessarily, the gossip/outbox model (NIP-65) tells clients which relays they should check:

https://mikedilger.com/gossip-model/

It is of course possible to combine this approach with undifferentiated access to well known relays, to fetch content from not followed people.