TOTP (thing that generates the 6 numbers every 30 seconds) whether that's a dedicated device (secure but very annoying) or a TOTP app on your phone (what most people use).
I at least have a different user account that only does TOTP but it isn't really a second factor if it is on the same device (since the idea is to make getting access to the code significantly more difficult than just getting access to the password).