You have to
1) create background task
2) gather the XML or JSON data and parse it (from the internet) and then put that data into the 'contentNode' of whatever object you are rendering. Final step is to set the contentNode of your component equal to the contentNode containing all of your gathered data.
this is quite a complex all-in-one example ...
https://gist.github.com/timbeynart-nbcuni/c73989ff347b101ef4176b8c104f82af
This one illustrates the super-simple contentNode principle for a video player
https://community.roku.com/t5/Roku-Developer-Program/VideoExample-playing-from-json/td-p/507410
This one illustrates how to do it if all of the json data was already held locally and was static
Another one where data is held locally
Official docs (uses XML rather than JSON)
https://developer.roku.com/en-gb/docs/developer-program/core-concepts/downloading-server-content.md
https://developer.roku.com/en-gb/docs/references/scenegraph/control-nodes/contentnode.md
https://github.com/rokudev/samples/tree/master/ux%20components/control/ContentNodeExample
