How does an authenticator app work?
An authenticator app is usually installed on a smartphone or other device and generates a random passcode every 30 seconds. The passcode can be used for sign-in, two factor authentication, an more.
Passcodes generated by an authenticator apps are generated from a secret code that is shared between the service you are using and your device, as well as the current time.
Are there alternatives to authenticator apps?
Yes, there are alternatives. Authenticator apps provide a layer of security and are a convenient option for use by many, but they are still vulnerable to phishing due to the 30-second window.
Security keys, like the YubiKey, are considered to be both more convenient and more secure. Yubico also provides a to use in conjunction with the YubiKey