Dock moves to either bottom of main screen OR left of second screen - can't do left of main screen

I'm running macOS Catalina on a Late 2014 Mac mini. Up until now things have been fine with the Dock (apart from it moving around unexpectedly sometimes). However, when I started it up again, the Dock was anchored on the left of the secondary monitor (connected with VGA to MiniDisplayPort adapter). I want it on the left of the main monitor (connected with HDMI). I turned off Separate Spaces in the Mission Control options and logged out and in again, but the Dock was still on the left of the secondary monitor. I unplugged the secondary monitor, put the Dock on the left side of the main monitor, plugged the secondary monitor back in, and the Dock went back to the left of the secondary monitor.

How can I make the Dock stay on the left of the main monitor?

Also, when the Dock is at the bottom of the screen, you change its location by moving the mouse pointer to the bottom of your screen. What about when the Dock is on the left or right? Do you do the same except move the pointer to the left/right of the screen? What if there is an external monitor connected on that side?

It would be so much easier if Apple just added a Dock control to the Views Display that you could move like you move the primary menu bar around.

The Dock will only go to the left of the left-most monitor, or the right of the fight-most, [based on your setup in System Prefs > Displays > Arrangement & not on their physical placement if this does not match.]

It will not sit in the middle.
If you want it to go to your current working display, it has to be set to the bottom, & you need 'Displays have separate Spaces' switched on.