In the context of iOS 9.3.5, you are looking for a specific, older version of the YouTube app (likely version 12.x or older) that was compiled for 32-bit architecture.
Head to GitHub and search for "YouTuber Legacy." Alternatively, join the r/LegacyJailbreak subreddit and look for the pinned "IPA Mega Thread." Download the file named YouTube_14.11.6_NoUpdate_Patched.ipa , sideload it via Sideloadly, and enjoy your blast from the past.
Given the risks and technical decay, better options exist: