Open AltStore on your iOS device, tap the icon, and select the YouTube IPA you downloaded.
Since the App Store won't serve the modern app to your device, you must download a legacy IPA (iOS App Store Package) from a trusted archive.
Installing an IPA on iOS 10.3.4 usually requires a computer or a specialized sideloading tool. Method A: Using AltStore or Sideloadly (Recommended)
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