Example of JSON response from API?

Would anyone be willing to share what the API response looks like for a relatively complex site? Custom fields, pages field, etc? I'd like to understand what the JSON looks like to determine if it meets my needs. Thanks!

Which API are you talking about? I'm not quite sure how this relates to Grav

Grav has a built in API, correct? I'd like to understand what the response looks like when accessing a page with custom fields, etc. with the API.

Are you referring to this?
That's not an API you can make cURL (or whatever) requests to per se (to my knowledge).
I think you'd need to create a plugin to be able to make requests and get JSON responses.