Not a able to get expected result with apis

[Arjun S Panicker]

Hi Team , 
can anyone help us with the following queries

1. On getting userlist details from google ads using rest api we are not able to get the audience size for gmail and youtube. At the same time we are able to get search and display sizes. Is there any api that is available to get the gmail and youtube audience size. 
2. Also what is the difference between Ad Destination type & Ad Network type.
   
3. How can we get the metrics for a campaign in different channels like youtube