Answer: By allowing access in the menu of the browser
The first time you need your camera, your browser requests access to the camera of your phone or your webcam. This usually happens as a pop-up in the middle of your screen or somewhere at the top of the window. If you click Accept, the browser can access your preferred camera or webcam and you can use it to verify your ID.
If you have denied or ignored this message, it's still possible to access the camera by refreshing the page on mobile or clicking on the camera icon at the top of the window. This process can differ per browser and device.