What is the username of the macOS recovery?
macOS recovery doesn't really have users or passwords. Well, technically it does have one user account: root. But Recovery's root account doesn't have a password; when you start in Recovery mode you're automatically logged in as root.

If you need to enter a password to get into Recovery mode, that's probably a firmware password. If a firmware password is set, you need to enter it in order to invoke any alternate boot mode (such as Recovery). This password has nothing to do with any user name or account, and has no use after the computer has started up.