As someone who has worked on a similar product, I would imagine they only store a token given to them by their payment gateway. The actual CC information is held by the PCI compliant payment gateway, while Docusign can use the token to charge a card without storing compromising information.
So if I'm understanding you correctly, you're a junior dev who wants to have a mentee? Shouldn't you be more concerned with finding a mentor at this point in your career? I suppose you could start with mentoring college and high school students who are interested in entering the field. Just my two cents