How do I turn on two-factor sign-in?
An authenticator app, plus recovery codes for the day the phone is gone and nobody can let you back in.
Settings → Security. Under Two-factor sign-in, scan the code with an authenticator app (1Password, Authy, Google Authenticator and the rest all work), type the six digits it shows, and it is on. After that, signing in asks for the app as well as the password.
Save the recovery codes. They are shown once, when you turn it on. A lost phone with no codes means a lost account: there is no other way back in, because a support desk that can switch it off for anyone can switch it off for anyone pretending to be you.
Each code works once. You can see how many are left, and make a new set, on the same page. Making a new set voids the old one.
Turning it off asks for your password. Signing in with Google is unaffected: Google is already doing this for you, and it does not ask twice.
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