Do not download random .IPAs from forums unless you trust the source. Stick to known archivists like MTMDev or Psychotext for patched legacy builds.
Disclaimer: This article is for educational purposes. Sideloading apps may violate Apple’s Terms of Service. Proceed at your own risk. Youtube.ipa For Ios 9.3.5
Even though these methods can work, they might stop working at any time due to changes in iOS, YouTube's terms, or app updates. Do not download random