Can a custom made checkout process trigger abandoned carts?

I am trying to work with abandoned carts and according to the docs it has to match this criteria

Carts are considered abandoned when these conditions are fulfilled:

  • The customers has added items to cart.

  • No changes have been made to the cart for at least an hour.

  • Wix Stores has the customers’ email address (either because the customer has logged in to your site or because they started the checkout process and entered an email).

But unlike the wix provided flow of checkout which I assume is storing emails on incomplete checkouts, I have no idea how a custom designed cart and checkout can ‘store’ a visitors email without the visitor logging in…

Does it have something to do with the field?

Is this even possible using the existing API?