A lost device is two problems: the hardware, which is replaceable, and the accounts signed in on it, which may not be. Preparation targets the second.
Three settings do most of the work: a strong screen lock, device encryption, and the platform's find-my service enabled with location sharing. Together they turn a serious breach into an inconvenience.
Speed matters after the loss. Marking the device as lost from another device locks it, displays a contact message and blocks payment features, and it works even if the device is offline when the request is made.
Afterwards, change passwords for accounts that stayed signed in and remove the device from your account's device list.