How do I find my IOS app's archive file?

They are stored here:

~/Library/Developer/Xcode/Archives

In that folder you will find a bunch of folders by date.

Alternatively, open XCode, go to Window -> Organizer. Right-click the archive you are looking for and select "Show in Finder"

Using Window->Organizer, you can select your archive in the list and choose the export option. Then export as save for iOS App Store Deployment. Once the process is done it will allow you to save the exported file as a folder that contains your ready IPA file.

However, it says I need to find my app's "archive" file. I believe that is the same thing as my app's .ipa file.

You need to unbelieve that. You can make an .ipa file from your app's archive, but you don't usually need to do that any more.

Okay, so, step one: choose Product -> Archive. That builds the archive.

Step two: There might be no step two, because the first step might take you to the built archive. But if not, choose Window -> Organizer -> Archives. There's your archive, and the upload process can be performed directly from this window.