Default character encoding for each operating system?

[Costello, Roger L.]

Hi Folks,
In a book that I am reading [1] the author mentions "the default character 
encoding for the operating system." What is the default character encoding of:

-          Windows 10

-          Mac OS

-          Linux

/Roger
[1] Practical Common Lisp by Peter Seibel, p. 165 (footnote 2).