There are hundreds of different Unices out there. Here is a - highly simplified - overview. Linux is just one branch of it, just a rather popular one.
These guys made a surprisingly complete chart - should give an impression on what this is about.
Linux (0.01) surfaces 1991, some 20(!) years after the first UNICS.
@Zac67 I had forgotten that diagram and it sums up the relationships better than I could in a few thousand words.
The important thing to note in this diagram is that most other modern Unix's are in parts based on the versions that came before them. Linux was build purely as a work alike and its code base is completely separate from any other Unix. (remember we are taking purely about the OS kernel internals)
Moral: If it looks like a duck and quacks like a duck, it probably an open source copy of a duck but not really a duck at all
Gaz
