The search term "YouTube IPA for iOS 9.3.5" typically refers to users attempting to install a modified or older version of YouTube on an older iOS device (specifically those running iOS 9.3.5, such as the iPhone 4s, iPad 2, iPad 3, original iPad mini, and 5th-generation iPod touch).
Since the official YouTube app requires iOS 12 or later, users on iOS 9.3.5 must use alternative methods to access the service. youtube ipa for ios 935
Here is a useful write-up regarding YouTube IPAs for iOS 9.3.5, including the risks, the alternatives, and the installation landscape. The search term "YouTube IPA for iOS 9
Sideloading an IPA on iOS 9.3.5 is not a "tap and install" process. You will need: Sideloading an IPA on iOS 9
AltStore is another tool similar to Cydia Impactor.
No Fix for YouTube’s Backend Changes
Even with the IPA, YouTube’s server-side updates (polymer redesign, ad system, captions, shorts) won’t work. The app will load but show errors.