The first method is to use the built-in Screen Time feature. This feature allows you to set time limits for certain applications, as well as block certain apps entirely. To use Screen Time, go to Settings > Screen Time and enable the feature. From there, you can set time limits and app restrictions.
Another method is to use a third-party app. There are several apps available on the App Store that allow you to lock specific applications with a passcode or Touch ID. Some popular options include AppLocker, BioProtect, and Lockdown Pro.
If you don't want to use a third-party app or the Screen Time feature, you can also create a folder on your home screen and move the apps you want to lock into that folder. Then, move that folder to the farthest screen on your home screen, effectively hiding it from view. You can also use the Restrictions feature in Settings to disable access to certain apps, but this feature is limited and does not work for all apps.
By using one of these methods, you can easily lock your iPhone applications and protect your privacy and security.