Questions about Lemmy

Not sure if this is the right place for this, still getting used to lemmy! I'm a bit confused on some things and I'm hoping people can answer for me.

• What happens to the communities/comments/accounts if a lemmy instance goes down? Do they just disappear?

• Can people on other instances use your username? Could others tell which is which in comments/posts?

• How can people afford to host an instance? Aren't there costs to hosting a server?

• Is there anything stopping corporate interests hosting a lemmy? I fear that these corporate instances will be the only ones that can handle large traffic and we're just back to Reddit.

• Can an instance go from fully federated to partially without telling it's users? How would they know?

Thank you for your answers!