Querying users and applications which exist on two different subgraphs

Hello, having a situation i can’t wrap my head around with knowledge i have so thought would post question here.

Create a gateway using apollo federation which will expose a data structure like below

 id: string;
 tracking: string;
 firstname: string;
 email: string;
 region: string;
 applications:{ // this is an array , so multiple applications per user
  company: string;

What i already have created
I have created two subgraphs. One for the User entity and another for the Application entity.

User Subgraph

  1. Created a resolver that takes id and tracking as parameters to find a unique user. So i also denoted these two fields as Key

Application Subgraph

  1. Created a resolver that takes User as input and outputs list of all applications by that user. To find applications for the user, i will need users first name, email and region.

I looked at various existing example code and they are very simple like just having ID as keys between fields. But in my case i will need to pass whole User object which is output from querying User subgraph as input to get applications. I don’t even know if this is possible, Please shed some light

so when i looked at rest example of federation, there is a gateway and below code in posts resolver.

What does the code actually do ? does it forward request to gateway, such that using the typename the gateway will now route the request to Users subgraph to query user information using the passed ID parameter ?