Nothing. Not a single thing.

There are a couple of websites that have you pay to use them, but most websites people use are either websites that are run by their friends, or free well known websites that the whatever nostr based program you're using knows already without you telling it about more websites.

Kind of like how most people play on the Minecraft server that's on minecraftservers.org, but you can join your friends Minecraft server if you know about it.

You know I think if you just jump into it, it'll make sense. astral.ninja for web browser and nostros for Android have been good to me.