If the subscriptions are of the same type, then - yes, they get aggregated, but if the subscriptions are different - they do not.
For example, you have an active Premium subscription for a month, it is valid for another 20 days, and you decide to subscribe to an annual Premium subscription. In this case, the subscriptions are aggregated, and after paying for the annual subscription, you will have a valid Premium subscription for 385 days.
If subscription types are different, they will be active at the same time and the validity periods are not aggregated.