Your credits will expire on Dec 19th, whether you have completely utilized them or not. After that, you will be charged on your credit card for the resources you use. Since you had $200 free credits, the services you used with those credits till Dec 19th are free and will not be charged on your credit card.
If you set up a credit card as your payment method, the payment is charged automatically within 10 days after the billing period ends. On your credit card statement, the line item would say MSFT Azure. You can see your billing period in the invoice as shown in the example below.
Here is the detailed document to help you review your individual Azure subscription bill : https://learn.microsoft.com/en-us/azure/cost-management-billing/understand/review-individual-bill
I would recommend going through the document shared above. Let me know if you have further questions.
You can also create a support request if you have questions or need help.