The forms in the donation platform are built using ActionForm. This module connects the website with the payment provider, for instance.
Buut it can do so much more. You can create any type of form, with any amount of fields, dependencies, dynamically built option lists, add unlimited actions to submit buttons etc. This is how you can not only make a payment but send a mail to the donor, yourself, submit to the news letter and present a thank you text.
This modules has been crafted by a company called DNN Sharp and it is not open source. Tjep's digital agency has a continuous subscription that we offer to charities to use for free.In case you prefer to host the platform yourself, you can purcahse a license starting from 150 USD.
More info here.