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So you finally installed YouTube on iOS 12.5.7. Is it the full experience? No. Here is what works and what doesn't.
If you try to download the app and it tells you iOS 13 is required without offering the older version Install Youtube Ios 12.5.7
If you see "Open" instead of a cloud/download icon, you already have it installed. If you deleted it, go to the App Store > Account (top right) > Purchased > My Purchases > Search for YouTube > Tap the cloud icon. So you finally installed YouTube on iOS 12