How do I sign out everywhere?
Open Account → Profile, scroll to Security & data, and choose Sign out everywhere next to All devices. You'll be asked to confirm, then every phone, tablet and browser signed in to your account is signed out — including the one you're using. Nothing else changes: your data, your plan and your Google connection stay exactly as they are.
Use it if you've lost a device, signed in on a shared or borrowed computer, or you just want a clean slate. It works whether or not you have a password, so it's the one to reach for if you sign in with Google.
What each option does
Sign out everywhere (Account → Profile → Security & data) ends every session on every device at once. Each device has to sign in again next time.
Log out (Account → Profile) ends the session on the device you're using — that one device only. Your other devices stay signed in.
Sessions also expire on their own after a period of inactivity, so an old device you never signed out of stops working eventually either way.
If you want your mail and calendar cut off too
Signing out doesn't unlink Google. To end Lonzo's access to your mail and calendar as well, use Account → Connections → Disconnect — and if you want the autonomous work stopped but the connection kept, Account → Automation → Pause everything does that instead.
See also: How do I reset my password? · How do I disconnect Google? · How do I pause the assistant? · How do I delete my account?