How do I open an iOS app that I transferred to my Mac? I downloaded an app on my phone and transferred it to my MacBook Pro and I got a file of the app. Whenever I click on it, it says the app already exists in my iTunes library and do I want to replace it. How do I open the app so I can use it?
I think there is a misunderstanding here. An iOS app (from your iPhone) cannot run on your MacBook Pro.

If you want to use a similar app on your Mac, I suggest you check if there is a Mac version of the application you want to use. You can search on the Mac App Store or on the app website.