The latest version is macOS 11.0, named Big Sur. Previous versions were known as macOS 10 (until 2020), OS X (until 2016) and before that Mac OS X (until 2012). These are derivatives of NeXT's OPENSTEP operating system, rebuilt to resemble the classic Macintosh operating system.
The operating system on Macintosh computers before Mac OS X was known simply as "Mac OS System 9", being the ninth major version of the original Mac OS.
Using PC keyboards
When a PC keyboard is connected to a Macintosh via USB, the Windows keys act as the Command keys and the Alt keys act as the Option keys. The positions of these keys are reversed from the Macintosh standard, with PCs having the Alt keys innermost.
Within System Preferences, under Keyboard or Keyboard & Mouse (depending on system version), you can remap the modifier keys, and thereby swap Option and Command.
Changing keyboard layout