How to send requests to OCC API in OSF within an endpoint?

Summary:

In an endpoint in OSF, how do I "chain" API requests in one endpoint?

For e.g. I want to get collections together with category image paths. When I get the root category, images aren't returned (just a repository ID). I have to make another request passing through the child categories to list the category images. So I want one endpoint that first, gets the root categories, flattens the child categories, then passes that into list categories and that response transformed and returned for the endpoint.

I can't find an "import" in OSF to use for making API requests into the store API or any documentation doing the same, neither in the reference applications.