Both Apple and Google use a 1024×1024 app icon (or equivalent) for the store listing. Apple requires exactly 1024×1024 px for App Store Connect; Google Play uses a 512×512 high-res icon. Your app binary still needs all the smaller sizes for devices.
One source, many sizes
Start with a 1024×1024 (or larger) square image. From that, generate every device size: iOS Asset Catalog and Android mipmap. That way the store icon and all on-device icons stay consistent and sharp.
No transparency (Apple)
Apple's 1024×1024 App Store icon must not have transparency. Use a solid background. Our app icon generator takes your image and produces the full set for both platforms.