Hi,
The REST API documentation is available at
https://github.com/DSpace/RestContract
To find all the Items in a Collection, you'd use the search endpoint, and pass in the UUID of the collection as the "scope":
In general, if you are trying to find which endpoint to use in the REST API, you can discover it simply by using the User Interface. The User Interface uses the REST API at all times... so if you click a button or visit a page in the User Interface, you can see which REST API endpoints it uses to gather that data. In your browser, you'd open the DevTools and take a look at the "Network" tab, where you can see every call the User Interface makes to the REST API.