On android you can hold down the power button and then tap lockdown mode. Then it will require your pin/password to unlock again. Ideally have a password as your authentication method if you're concerned about privacy.
On newer Android, holding down power apparently just launches Gemini's AI assistant. You can change it back in settings, or just hold down power + volume up to get to the old menu that has the Lockdown button.
(Not to be confused with power + volume down which takes a screenshot).
