No, there are no instances, there are just caching servers called "relays" that are run by many different people.

You create the content on your device and then send it to many of these relays (usually 10-20) and other users pull the content from also 10-20 relays. So if one relay decides to block you, then people still get your content from other relays. If all relays decide to block you, then you can (quite easily) run your own relay and tell your friends to pull the data from it. You own your data and you can resend it to wherever you want (it's signed by your private key, so it's verifiably from you).

Because there are no instances. There are relays that you post to, and that people use to fetch notes from. But there are no “user accounts” on the relays. If the note is signed by your private key, it comes from you, regardless of how it came to me. It can be through a bunch of relays.

Relays can and do filter notes by pub key. To fight spam, and problematic content. But you as a user can always change the relay set that you post to. And, of course, host your own relay, which is pretty straightforward.