Why does `ls` work on my cmd? (Windows)

From cmd you can use the where command to find the location of an executable:

where ls

This is similar to the Unix which command, and like which it does not report internal commands and aliases. It also has additional options not in which.

If it's one of the Unix commands you have imported, you can use which itself.

You might have installed Git repository. This might be from your C:\Program Files\Git\usr\bin\ls.exe