YouTube Vanced was a widely used third-party modded YouTube application with features such as ad-blocking, background playback, and SponsorBlock. The project was terminated, though. In case you're interested in downloading YouTube Vanced, you might need to consider alternatives such as ReVanced, which is an actively developed fork. To get it installed, download the ReVanced Manager from its original GitHub or safe sources and then patch the native YouTube app with the preferred functionalities. Note that sideloading third-party apps is a risk to security, so always confirm sources and work with reliable communities such as XDA Developers.