Hi Alexis,
Creating a form like that is not something our plugins can really do out-of-the-box.
Very extensive custom coding would be required to modify our plugins to be able to support that kind of form. Unfortunately we cannot help with custom coding, or providing insight on what they did to achieve their own custom forms. Please read more about this policy here: https://theeventscalendar.com/knowledgebase/what-support-is-provided-for-license-holders/
It looks like they may have created an event and added tickets using Event Tickets Plus “Attendee Information” features, which you can read about here: https://theeventscalendar.com/knowledgebase/collecting-attendee-information/
But then they modified code to make it so that only one set of attendee information was collected, and the form elements were “expanded” by default.
I would recommend contacting the folks behind this site and asking them for insight into their customizations. They may be able to describe what they did in detail.
Best of luck with your project,
George