In Xcode, app icons live in Assets.xcassets / AppIcon.appiconset. You need multiple sizes: 20pt, 29pt, 40pt, 60pt, 76pt, 83.5pt, and 1024pt, each in 1x, 2x, and/or 3x as required. The Contents.json file tells Xcode which file is which.
Pixel dimensions
For example, 20pt at 2x = 40×40 px; 60pt at 3x = 180×180 px; 1024pt = 1024×1024 for the App Store. Getting the list right by hand is tedious. A generator that outputs the full AppIcon set plus valid Contents.json saves time and avoids submission issues.
Drop-in ready
App Asset Generator produces the exact folder and file structure Xcode expects. Add the AppIcon.appiconset to your project's Assets.xcassets and you're done—no manual resizing needed.