Fundraise Up incorporates Stripe's protections, ensuring that if a donor is paying with a card that requires 3D Secure/SCA authentication, that they are prompted appropriately for any additional info.
To ensure that Strong Customer Authentication (SCA) and 3D Secure protections are respected and used on card-based purchases, your Fundraise Up donation checkout overlays Stripe's authorization modals overtop the donation checkout in any case where a donation requires additional verification info from a donor.
When a donor clicks the button in checkout that finalizes their actual transaction, if their card requires 3D Secure authorization, they'll be shown that additional popup.
Donors (and you) can easily see which button finalizes their transaction because it has a distinct label: Donate X (where X is the currency and amount of their donation). So, they'd see the 3D Secure popup if applicable after clicking the button labeled Donate X.
All other pages in checkout have either a Continue button or a more specific label on the buttons, indicating what actions are available.