If you have an iPhone 6s or newer, you cannot downgrade iOS. Apple stopped signing older iOS versions years ago. This is not a viable path.
If you have ever downloaded YouTube in the past using your current Apple ID, you can often download a compatible older version directly from Apple's servers. Open the . Tap your Profile icon at the top right. Select Purchased (or "All Purchases"). Search for YouTube . youtube ios 125 7 download apk old version top
Google frequently updates its security protocols. An app version as old as 125.7 may no longer be compatible with Google's current login servers. You might successfully install the app, only to find that you cannot sign in to watch videos. If you have an iPhone 6s or newer, you cannot downgrade iOS
