Re: What is that depth information used for?
I mostly work with iOS but this is my field,
iPhones with dual cameras will use 2 cameras to get depth of field, single camera iPhones. won''t do it.
In addition phones that support FaceID have a depth camera on the front facing camera.
And now there's LiDAR enabled phones too.
Android has the depth API, which can work with a single camera, it just requires a bit of movement so it can compare multiple frames to derive some depth data, from our experiments though, it is nowhere near as accurate as the iPhone. Some android devices also have time of flight sensors, which really improve the results.
For a single image though, you are correct, they will not show information that's behind other things, in which case you can build a model using say a photogrammetry tool, or now, with phones with depth sensors, you can now build a model in app.
But I suspect, even from a single image, there are probably tools that can reconstruct missing background information with a bit of AI, but it's not my field.