what to do if somebody have an idea to make or improve an application?

Is it an Apple app?

https://www.apple.com/feedback/

It depends on the person & who makes the Application.

Apple have this policy –

https://www.apple.com/legal/intellectual-property/policies/ideas.html

Whatever you tell Apple becomes their property. If you don't like the sound of that you would need to start a dialog with Apple.

Other Applications that are not made by Apple may have similar guidelines but you would need to contact the relevant developer.

You have to have a big enough reputation for Apple to call you:

https://www.apple.com/legal/intellectual-property/policies/ideas.html