How to get YouTube on older iPhones (iOS 12.5.7)
If you have never downloaded YouTube on your Apple ID, the app won't appear in your "Purchased" list. You can fix this by using a newer device. Borrow a newer iPhone or iPad running iOS 14, 15, or 16. Sign in to your Apple ID on that newer device. Download the YouTube app from the App Store.
Detailed feature content
"Unable to Download" or "This app requires iOS 14.0"
: Use the search bar to find YouTube in your list of previously downloaded apps. Install : Tap the cloud icon next to it.

