You cannot simply tap an IPA file on your phone to install it (unless you are jailbroken). You must use a "Sideloading" tool. Here is the standard method for 2024.
Open AltStore, navigate to the "My Apps" tab, and tap the "+" icon at the top right corner. Select the YouTube 12.1 IPA file you downloaded and follow the on-screen instructions. youtube 121 ipa download hot