What to do before switching mobile devices

Some things are not backuped when you backup your iOS device:

FaceID and TouchID
No way to backup.
Encrypt your iOS backup if you want to keep WiFi keys.

Authenticator
Either backup into icloud or manually check if your credentials are really in your key safe.
Look into your authenticator. For the apps/ services listed there update where your security codes are sent, either to an email or to the new device.

Signal
Read Signal: Backup and Restore Messages

Financial apps
Often banks send passwords or onetime tokens per mail. Consider contacting your bank before you switch devices.

Subscription accounts
Logoff from subscription accounts like Spotify, Netflix, etc before loggin in from the new device.

Remove SIM and SD Cards