How to Make the Dock Appear on External Display on Mac
Newer versions of Mac OS brought significant improvements to multi-display support for Mac users who have their computer connected to an external screen or two.
One of the more helpful feature changes with multi-display support is the ability to access the Dock on any of the connected screens to a Mac through the use of a simple onscreen motion gesture trick. Learning this trick allows you to quickly show the Dock on any display connected to a Mac.