Since the official app is unsupported, you can try these alternatives:
This last compatible version is incredibly old (likely dating back to around 2013-2014). It will be slow, lack countless modern features (like comments, dark mode, and shorts), and may have severe playback issues. Many of Google's core video APIs have changed since then, so you might encounter errors, be unable to log in, or have videos fail to play. It's still worth a try, but keep your expectations low. youtube for iphone 4 ios 712 download upd