Finding a YouTube IPA compatible with iOS 9.3.5 is difficult and risky. The safest path is to use the YouTube mobile website or upgrade to a newer device. If you consider sideloading an IPA, prioritize files from verified, original backups and understand the security and account risks involved.
When looking for a "useful feature" set for a custom or tweaked YouTube IPA on this version, legacy users typically prioritize: youtube ipa file for ios 9.3.5
This is the most stable method, as it signs the app with your own Apple ID. Finding a YouTube IPA compatible with iOS 9