Do you have to be an American Citizen in order for me to apply for a Apple Credit Card?

Citizenship is not required.

However, you must be a lawful U.S. resident with a valid physical U.S. address (not a P.O. Box). A military address is also acceptable.

How to apply for Apple Card - Apple Support

saneka242 wrote:

How would you pay monthly for the Apple Credit Card?

From a linked financial account, usually.

See the information available here: How to apply for Apple Card - Apple Support

And read the Terms & Conditions.