YouTube Vanced was a very popular modded YouTube that had ad-free watch, background playback, and other premium features. It is no longer available, so downloading it is not advisable. You can download alternatives such as ReVanced, an open-source fork. To install, download the Vanced Manager first from a trusted source such as GitHub or reputable forums. Next, follow the instructions in the app to install MicroG (for Google login) and YouTube Vanced or ReVanced. Watch out for scam websites that could have malware. As it is not officially supported, updates and security concerns need to be thought through before running such apps.