How does 3D Secure v2 work until its full deployment?

The payment gateway initiates the process of 3D Secure v2 authentication.

If the card is v2 3D Secure-enrolled, the 3D Secure v2 process continues.

If the card is not v2 3D Secure-enrolled, the payment gateway proceeds with cardholder authentication in 3D Secure v1 mode.

If the card is not enrolled in v1 3DSecure, then it continues the transaction without authentication but with the risk of having the authorization denied.