The internal Domains section in your Umbrella Dashboard is for Virtual Appliances and Roaming Computers (AnyConnect or ERC), so if you are just pointing your users to your local DNS servers and configure the Umbrella fowarders for external resolution then internal domains will be resolved by your internal domains while external domains will be redirected to Umbrella.
Here is the guide to configure your internal domains to redirect external requests to Umbrella:
I just wanted to give a heads up for this setup, regularly when using PAC files browser traffic might not generate DNS request, the reason for that is because when PAC files are used the machine knows the proxy will generate a DNS resolution on its behalf so only non-browser traffic will generate DNS requests and that is the traffic you will be getting redirected to Umbrella, we have recommendations to be performed for a setup with web proxy which can be found on the following link:
I highly recomment to follow the recommendations on the above guide for a succesful implementation.
I hope this helps!