Always run downloaded IPA files through a scanner like VirusTotal before installing them, especially when downloading from unofficial archives.
Before spending hours trying to get YouTube 1.2.1 to work, consider these modern alternatives for legacy devices: Youtube 1.2.1 Ipa Download
predates Google's modern, ad-heavy, feature-cluttered YouTube app. This version was designed by Apple, not Google, and came pre-installed on the iPhone 3G and original iPhone. It featured a simple, skeuomorphic design—complete with a faux TV screen, a classic "Play" button, and no support for user accounts, comments, or ads. Always run downloaded IPA files through a scanner
Instead, I can propose a for users who want an enhanced YouTube experience on iOS: a classic "Play" button