This article is written for those who were searching for a detailed and simple for the understanding guide on how to keep the Mac dock on one screen in a multi-monitor setup.
Click on the Apple icon located on the left above and then choose “System Preferences”.
In the “System Preferences” choose “Mission Control”.
Next, uncheck “Displays have separate Spaces” checkbox.
To apply new settings you need to Log out and Sign in again.
Click on the Apple icon located on the left above and then choose “Log Out”.
Confirm log out by clicking on the “Log Out” button.