How you are billed – Amazon Elastic Compute Cloud

The pricing benefits of Reserved Instances are shared when the purchasing account is part of a set of accounts billed under one consolidated billing payer account. The instance usage across all member accounts is aggregated in the payer account every month. … Amazon EC2 automatically applies any discounts to the part of your purchase that …

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/concepts-reserved-instances-application.html