Pressing Check, or hitting Enter in the box, sends one GraphQL request from your browser straight to Twitch's public endpoint at gql.twitch.tv, with no proxy in between, because that endpoint accepts cross-origin requests. The request asks for a single user record by login: the account id and canonical login, the display name, the creation date, the Partner or Affiliate role, and whether a stream object currently exists. The match is exact, so a name that differs by one underscore is a separate lookup and no near-misses are suggested. Before anything is sent, the box strips a leading @ and lowercases what you typed, because Twitch logins are lowercase.
The badges come from that same single response. Partner or Affiliate is read from the account's roles, and only one of the two is ever shown — Partner takes precedence if both flags are set. A “live now” badge means only that Twitch returned an active stream for that channel at the instant of the request, not a status someone recorded earlier, and no viewer number is fetched alongside it. The bold line is the verdict itself; the channel's display name appears in the sentence beneath it and can differ in capitalisation from the login you typed. Viewer counts, follower totals and past-name history are not part of this query.
Because the lookup keys on the current login, a “no channel with this name” answer means only that Twitch returned no public channel on that exact login at that moment. It reports what exists, not what you may register: this query has no way to see whether Twitch is holding a name back after a rename, after a suspension, or because it is reserved. Only Twitch's own signup or name-change form settles that.
There is no stored list of names behind this page and no cached answers. Every click issues a fresh request, so the result reflects the second you pressed the button and can change before you finish signing up. If the request fails or Twitch cannot be reached, the box shows an error rather than reporting the name as free, so a failed lookup is never rendered as availability. Names are checked one at a time, and the request carries only Twitch's public web Client-ID header — no login token — so only public channel fields come back.