When you plug in an external monitor, and you want to make it the primary monitor, it's super simple!
Open System Preferences. Click on "Displays"
(or just click on the monitor icon in your top dock. Choose "Open Displays Preferences")
Click on "Arrangement"
Move the white bar at the top of one of the displays to your preferred monitor. Now your dock and files will be on that monitor instead.
For full details, check out geekbeat.tv/tip12