jmap-bridge is self-hosted software that runs a JMAP server in front of an ordinary email account, reading and sending mail over IMAP and SMTP (and, optionally, contacts over CardDAV) so that JMAP-only clients can use that mailbox.
This instance is operated by the person who deployed it and serves only the mail accounts they configured. The JMAP API requires the account's access token; these informational pages exist for the Google OAuth consent screen.