Hello,
Welcome to Microsoft Q&A!
If you are asking a built-in UI that contains both functions like show taking a picture using the camera and selecting an image from existing files, there is no built-in API that could meet the requirement. UWP has built-in API for taking pictures using the camera. For example: [CameraStarterKit][1]. As you mentioned that you've implemented these functions before, you might need to combine these two functions in your project manually.
Thank you.