Okay, after a little research, I've got some answers.
@LarryJoe33, you do not have to add the funds to your Apple Pay Cash card first in order to send money to someone. It can be sent from a debit or credit card. However, money received will be added to the Cash card.
@Cscottrun, I just tested out your question and even though I have my AMEX credit card as my default Apple Pay card, it automatically used my debit card when I tried to add funds to the Apple Pay Cash card.
However, when you are actually sending someone money through messages, you will have the option to choose any card you like at the verification screen. If you have any balance on your Apple Pay Cash card, it will default to using that first.
Here is a link to the article which should answer any other questions you may have.
https://support.apple.com/en-us/HT207875