URL Path Based Routing allows you to route traffic to back-end server pools based on URL Paths of the request.
In the question there are two different path from where the traffic is getting generated as below
https://adatum.com/internal
https://adatum.com/external
So in this case we can use URL path-based routing feature of Application Gateway.

Reference:
https://docs.microsoft.com/en-us/azure/application-gateway/url-route-overview