The Build Host is bundled with Xamarin.iOS
Because the build host is bundled with Xamarin.iOS, you'll automatically have it whenever you install or update Xamarin.iOS.
How can I install Xamarin.iOS?
You can download an installer that will add all the Xamarin tools, including Xamarin.iOS, from your account's download page: https://store.xamarin.com/account/my/subscription/downloads
What if I want to install just Xamarin.iOS, or a previous version of it?
Those are also available on the downloads page. Select 'View All Versions' and pick the installer you need. More details in this guide: How do I downgrade to an older version of Xamarin?