Yes, it can scale to 1 billion users. No, not everyone will see all the same notes/posts.
Nostr is not a consensus protocol. Not everyone has to agree on everything or see every post.
Nostr is a decentralized and localized protocol. There is no front page of nostr because you only see posts from relays you’re connected to.
Just like villages on earth, relays on nostr connect local communities.
If the relay is beefy enough, it can support as many users as it needs