How do I keep the dock pinned to my Mac?
It keeps moving to my external Mac display.
MacBook Air (M1, 2020)
The Dock will appear on whichever display is set as Main Display. You can verify which display is set as Main Display in System Settings ➜ Displays. Click on a display image there, and look at the Use As drop-down.
It can also move to whatever Display your mouse pointer is currently on, if you move the mouse to the bottom of the screen and hold it there for a couple of seconds.
Avoid doing that, if you don't want it to move to that display.
You can return it to the original display by moving the mouse back to it, and hovering it at the bottom of the screen for a couple of seconds.
There is no option to turn this behavior off however. Just don't place the mouse pointer at bottom of the screen.