Adding custom data in the API response

Hello Question - I'm not really sure where to look. May someone point me in the right direction to find relevant docs?

I have a field

{
    name: 'location',
    type: 'text',
    label: "Location",
    required: true,
},

And of course with the field filled out shows up as this in the API

"location": "London",

For some basic example lets say I want to return a value but is not something seen or edited in the collection for this. Is hooks where I should go? I want the result to look like such:

"location": "London",
  "customHiddenStuff": "Foo",
  "customHiddenNumber": "123",
  etc

Where might I find docs relevant to this? Thank you