which is the best app lock I can use for my iPhone 8 normal?

I have never heard of such a thing on Apple devices. Something gives me the impression Apple wouldn't allow a third party app to take that level of control of your device. Read about Screentime. It isn't exactly what you describe but serves a similar function, plus it is already on your device.

I have never heard of an app lock. Maybe you mean this? Use Screen Time on your iPhone or iPad - Apple Support

What do you mean by app lock?