Navigate to Extension -> Manage Extensions and open the Manage Extensions.The steps below assist you to how to install the Syncfusion Xamarin extensions from Visual Studio Manage Extensions.
Visual Studio 2019 or lower Install through the Visual Studio Manage Extensions
You can either use the Visual Studio to install it or go to the Visual Studio marketplace to download and install it. In below Visual Studio marketplace link, Syncfusion publishing the Xamarin Visual Studio extension.