As far as I know, you need to share relays to see other people's posts. Some relays do propagate to multiple relays, but most are just silos.

More relays will bring in more posts for you.

Please, someone correct me if wrong, but:

You: Relay 1 & Relay 2

Friend: Relay 1

Company account: Relay 2

If you post to relay 1, only your friend sees it. If you post to relay 2 only the company sees it.

I have seen some clients that show a placeholder for a post made on a relay you don't follow, but it will say "This post is not available because you don't follow this relay".

For example, you reply to the company on relay 2, but post your reply it to all your relays, it's kinda like a snippet of the conversation "floating" on a relay you share with your friend. I think work has or is being done on this, but I'm not sure how it works exactly.

Hopefully I'm not completely wrong in that last part.