YouTube Studio- SmartPhone App helps in managing your content, account and other customisations. 

Google PlayStore - https://play.google.com/store/apps/details?id=com.google.android.apps.youtube.creator&hl=en

App Store- https://apps.apple.com/us/app/youtube-studio/id888530356

YouTube itself gives Background MUSIC that you can use in the Videos you are uploading to YouTube.

Audio Library can be found in your YouTube account- Studio. Studio.youtube.com 

Audio Library: https://studio.youtube.com/channel/UCX3r1r35oQoPtSyiQvXy37w/music

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