Function describe_server 

async fn describe_server(__arg0: State<AppState>) -> Response

GET /xrpc/com.atproto.server.describeServer

Discovery: what this server is, before a client has any account on it. availableUserDomains is read out of the registry’s real zone configuration rather than stated as a fixed string, so a deployment with several zones reports all of them — see Registry::available_user_domains. Every other field the lexicon declares optional is left off rather than answered with an invented value; see DescribeServerResponse’s own doc for which and why.