I've taken some photos and made a composite image that is 4160 pixels wide which shows the above effects.
There are 4 slices taken from four photos. From top to bottom: 4:3 in camera app, then again in the Open Camera app, then 16:9 in the camera app, then again in the Open Camera app. In both cases the Open Camera app seems to have a slightly wider field of view than the inbuilt app, even though the final image pixels are the same. I've right aligned everything so the bench is on the right. If you look at the bush on the far left of each strip you can gauge how much is showing.
There can't be a 1:1 correspondence between the output of the camera and the final file that makes it to storage. The two apps have different fields of view too. I'd expect the 16:9 image to be slightly wider in pixels than 4160.
Another example, use the front camera and take a photo of your face at arm's length. Do this for 16:9 and 4:3 sizes. Then look at the images 1:1 on your screen or computer. Surely your face (in the central portion of the image) should be the same size? It isn't! the 16:9 image is smaller! Why? Why through away detail by selection 16:9? Has someone typed the wrong numbers/aspect ratios into the driver files?
Here are some photos of a tape measure. The 16:9 shots seem to have an extra millimetre visible on the scale, but this wider real life image is imaged over 3840 pixels, whereas the 4:3 images have a slightly smaller field of view horizontally but spread over 4160 pixels. Why is quality reduced on the 16:9 shot?
4:3 Nokia Camera
4:3 Open Camera
16:9 Nokia Camera
16:9 Open Camera