How do I stop someone signing in as me?

Turn on two factor. An admin account has to, because it can reach everything.

2 steps · about 3 min

1. 01

   Scan the code with an authenticator app

   Any of them work: Google Authenticator, Aegis, 1Password, Bitwarden. Open Settings, then Two-factor authentication, and point your phone at the square.

   
2. 02

   Or type the key by hand

   Switch to Enter a key if the authenticator is on the same device, or you keep your codes in a password manager. Then type the six digit code it shows you and save the recovery codes somewhere that is not this device. Each one signs you in once if you ever lose the phone.