It's literally called "Billing" and the "Cost Explorer" is the first item in the navigation when you go to the page. I've attached a screenshot and highlighted it for you. FYI, this month is the first time I've opened the AWS GUI in 4-5 months, and it took me about a minute to find the billing page and drill into our AWS costs. I've provided an image that shows how to get to the cost explorer from "Billing" (you can use the search box available on the AWS account landing page to type in "Billing" in order to access the billing page of AWS).
EDIT: sorry, I forgot to include the screenshot link - https://imgur.com/a/tVFoocr