A 2FA authenticator generates time-based one-time passwords (TOTP) that you enter in addition to your password when signing in.
Open the Accounts tab and tap the plus button. Scan a QR code provided by the website or service you are protecting.
TOTP codes refresh every 30 seconds. This is expected and helps keep your account secure.
Check that your device time is set correctly (automatic time). If the issue persists, remove and re-add the account using the service recovery steps.
If the service allows it, you can add the same account to multiple devices by scanning the setup QR code on each device. Some services show the QR code only once.