Do non authorised repair shops steal original parts?

My iPhone 6S had some issues a while back with all its physical buttons such as volume and the power button. I dealt with it for a while until my phone completely shut down and I had to get it repaired. 

I took it to the nearest repair shop that offered to fix it for around $60 to $75 aud. Which is pretty cheap.

they fixed it for me in less than a day and I when I went to pick it up I checked all the buttons and data and I was happy with the results. 

however I forgot to check other things, which I began to notice a couple of days later. The main thing was my rear camera. 

After getting it repaired my rear camera looked normal at first but when I went to take a picture, it was not working at all. The screen was completely black and did not show anything that the camera should be capturing. The front camera was fine but the back one was totally useless. 

I looked closer to see if anything was blocking the camera and I was surprised to find dust inside the lens of the camera which had never been there before. I make an effort to keep my phone very clean and I would have recalled if there was dust INSIDE my camera lens. 

however a bit of dust would not stop the camera from taking a picture at all. It might be a bit fuzzy but you would still be able to take some sort of picture. Mine though could not take any pic.

I also noticed a small dent in the metal part of the lens which was not there before. 

Im beginning to think that my original camera was swapped while getting my phone repaired. 

if anyone knows any other explanations for this please let me know.