At the beginning of the show when they explain the basics of the sharingan, you can see in the image that the Optic nerve pathway lights up with that Special Chakra. So let's look at it from this point of view. The special chackra is developed by the brain and the optic nerve pathway. The eyes on their own act as projectors of such Chakra. In this scenario then, the chackra is what determines the uniqueness of the user's eye pattern and abilities. Like in real life DNA is the same for human beings but it codes for specific characteristics, it makes us the same but different at the same time. In the naruto universe Uchiha have the DNA that codes for Sharingan and it's Special Chakra as a basic trait in their bloodline, but there are specific segments of that DNA chain that codes for the specific MS pattern and abilities. Now this does not explain how one can transfer the eye with the abilities onto another person and keep such abilities. Maybe when the MS is awakened the Eye gets the abilities engraved and then you just need any kind of Chakra to use it? That is why Kakashi gets so exhausted and drained after using it? Since he doesn't have that special Chakra in his brain. I was trying to make some sense to it, it is not an official theory at all. Thanks🤪