X509Chain.Build() method explanation

I want to validate chain of certificates, I get a X509Certificate2 collection and have to validate if all the certificates build one chain.

Usually, in order to verify the certificates chain I should take the digital signature from the leaf certificate and check if it is signed by the root certificate - but in .NET I can't find a way to extract the signature from the X509Certificate2 object.

Therefore, I thought of using X509Chain.Build() method in the following way:

   void ValidateChain(X509Certificate2Collection collection, X509Certificate2 leaf)
    {
        X509Chain x509Chain = new X509Chain();
        x509Chain.ChainPolicy.ExtraStore.AddRange(collection);
        bool isValid = x509Chain.Build(leaf); 
    }

But I have some questions about the build method:

1. As I understood, the chain was built also from my computer store, and I want that it is built only from the ExtraStore, how can I define this behaviour?
2. I saw that after the chain was built it doesn't contain the Root Certificate; my question is why, and how can I verify that the chain has Root CA, since this is not part of the chain elements.

I will so appreciate it if someone can explain to me how the Build() method works.