You can use VirgilCardVerifier without additional parameters passing to constructor only CardCrypto. Signatures will be verified only from Virgil Service and card’s owner. Other parameters you can use if you sign cards with additional keys.
In this case
PUBLIC_KEY_STR is a public key, which you have to use for signature verification in base64 string. And signer is a string-key which authenticate a signature, which should be verified.
Virgil SDK allows you to manage the signature verification using Card Verifier. Detailed information how to set up it you can find here: https://developer.virgilsecurity.com/docs/go/how-to/setup/v5/setup-card-verifier
More information about Virgil Card Structure you can find here: https://developer.virgilsecurity.com/docs/api-reference/card-service/v5#card-structure