There are different ways to put a password on apps depending on the device and the app itself: For iPhone/iPad:

• iOS 18 and later allows you to lock apps using Face ID, Touch ID, or a passcode by tapping and holding the app and selecting "Require Face ID" or "Require Passcode." This feature prevents accessing the app without authentication.

• Additionally, built-in features like Screen Time can create app limits with a passcode to effectively lock apps.

For Android:

• Many Android phones have a built-in "App Lock" feature in settings where you can enable it, set a pattern, PIN, or password to lock certain apps.

• Third-party apps like Norton App Lock or AppLock can also password protect apps using PIN, pattern, or fingerprint.

For specific apps (e.g., WhatsApp):

• Some apps like WhatsApp have built-in app lock settings where you can set a password or biometric lock on the app itself.

For accounts with 2-step verification (e.g., Google, Microsoft):

• You can create app passwords for less secure apps or devices that don't support 2-step verification. These are special 16-digit passwords used for individual apps.

In summary, the method depends on the device, operating system version, and app features. For most phones, you can find app lock in device settings or use third-party locking apps. Some individual apps also have built-in locks. If a more specific guide is needed for a particular phone model or app, that can be provided.