They need to authenticate the 3DS. How that's done is entirely on their bank. Some are done with a text message, through banking app, etc.. Currently the payment is in a pending state waiting for them to finish 3DS. Once they do Stripe will push the payment through and notify CBSubs to complete the payment.
We're using their Stripe Elements API below. This doesn't require us to do any manual handling of 3DS. Stripe is supposed to be handling it and appears to be doing so in my tests.
stripe.com/docs/payments/payment-element
Below shows the workflows that the user should have experienced.
stripe.com/guides/3d-secure-2
I recommend using Stripe Checkout if you require SCA though (e.g. accept payments from India) as it can handle the workflow better in an isolated environment and is directly recommended by Stripe to do so for anyone requiring SCA.
I will review if there's anything further we're supposed to be doing immediately though as it's possible we missed a test case, but I had already tested every card type that Stripe provides for testing.