Xamarin is targetting .NET standard, and not .NET framework, so most of the libraries you are using will not be compatible with your app
Since i’m currently working on the exact same thing as you, here are a few advices :
- There is no package that will download your youtube wideo/audio from Xamarin, I never found any of them targetting .NET standard
- If you really want to download the video from your app itself, I recommend using React Native instead of Xamarin, a lot of NPM packages can download youtube videos from your app, like YT-DL
- If you really want to use Xamarin, you will need to make an external server and access it through the download manager of your app, or a custom web request. You can use the same package I mentioned before if you’re using node.js
You can see my experimental app working here: LocalTube
Try giving more details in your future questions
solved How do download mp3 from youtube to android using Xamarin.Forms/C#?