An example project showing how to use a background URLSession to keep downloads going even when the app is terminated as shown in this post - https://williamboles.com/keep-downloading-with-a-background-session/
This project uses TheCatAPI to populate the app with downloaded images. TheCatAPI has an extensive library of freely available cat photos which it shares via a JSON-based API. While free to use, TheCatAPI does require you to register to get full access to it's API (limited access is provided without an API key). Once registered you will be given an x-api-key token which you can paste as the APIKey value in NetworkService so that it is sent with each network request.
If you have any trouble getting the project to run, please create an issue.