iPhone and iPad use different app icon dimensions for different contexts: Home screen, Spotlight, Settings, Notifications, and the App Store. Sizes range from 20×20 (pt) up to 1024×1024 for the store.
iPhone
Common sizes include 20pt (1x–3x), 29pt, 40pt, 60pt, and 1024pt for the App Store. Each has a specific pixel size at 1x, 2x, 3x.
iPad
iPad adds 20pt, 29pt, 40pt, 76pt, and 83.5pt (for iPad Pro). The 1024×1024 asset is shared for the App Store. Use an Asset Catalog–compatible generator so all sizes are correct for both iPhone and iPad.