Microsoft originally developed Xenix which was their version of Unix. They eventually sold it of to SCO but retained some financial interest in it. I’m not sure if they still have any financial ties to it or not. Microsoft was still using Xenix for some internal development many years ago. I’m not sure if anything from this has survived to the present day but I am guessing probably not.
Microsoft often gobbles up other companies, and they get whatever comes with those other companies. For example, when they bought Hotmail it was running some variant of Unix (I don’t remember which one). Microsoft went to great expense to convert all of Hotmail over to Windows Server instead of Unix so now Hotmail is running Windows.
Along similar lines, Microsoft bought Fast Search and Transfer a couple of years ago, which had Windows and Linux/Unix versions. Microsoft has continued to support the Linux/Unix versions but fairly recently announced that they would be dropping them soon.
Microsoft also makes Microsoft Windows Services for UNIX, which is basically a Unix subsystem for Windows. The main purpose of this is to allow Unix programs to be more easily ported to a Windows environment.
There are probably a lot of other things that I am not aware of.