how 3d-secure flow works if merchant is not enrolled to 3d-secure but a cardholder does and vise-verse

Question

Im implementing 3d-secure in my payment flow. but i have query related to the flow

1. If a cardholder is enrolled with 3D-Secure, but the merchant is not, how does the payment flow work?
2. What if the merchant is enrolled with 3D Secure, but the cardholder is not? Specifically, will there be a redirect?

i read stripe documentation and few others documentation. but couldn't be able to findout answers for my queries.