App Lock!

Individual Apps can only be locked if the developer of the app enables that feature in their code. iOS itself does not allow locking of apps, but does allow locking the entire device, see - https://support.apple.com/en-us/HT204060

iOS gives developers an ability to use Face ID or Touch ID for locking the apps. If they do not implement this, contact the developers of the app you want to protect and ask them to do so.

I don't understand why do you need a lock on an app when you are the only one who can unlock your iPhone.

thanks! yeah you're right iOS doesn't provide but they have to because its a important feature :)

No. They don't "have to".

App developers have to code that functionality into their apps.

You have the ability to lock the phone. Don't let people you don't trust use it. It's not intended to be a multi-user device.

While individual apps may require a passcode, the only way to protect everything is to set a passcode to unlock the phone and don’t share your phone with someone you don’t completely trust.

There's no such app or I wouldn't trust any.

iOS can not provide a lock for individual apps. That is up to the developer of the app to do that.