Is it best to leave the bird outside without the battery to adjust to the (20f or below) cold before a flight or just fly it right away?  Which way is best?

No! The internal components (and battery) will warm during normal operation. So, IMO, it's best to start closer to their normal operating temperature.

In my case, I let the Mavic get exposed to ambient temperatures while I hike to my flying location (this is usually several hours). This acclimates the camera and gimbal and seems to avoid the video jello/jitters that I will otherwise get. Of your two technicques, I'd think just letting it sit without battery for a while would be the way to go but I have also flown and noticed that the jello will go away in the latter part of the flight.  I am sure it depends on the temperature differential how long you need to let it sit.

Hi Davey,
There's a number of posts here on the subject - but from my experience I just take it out when I'm ready to fly, start it up and fly. I have flown in weather as low as -32C (-25F) without any issues except for cold fingers. The battery was at +25C (77F) after the flight.

I have never ran into issues with the lens fogging up either.

Get out there, fly your drone and have some fun!

I had an alarm one time, something like "Battery Temp low, please warm before takeoff" (something along these lines).
My spare batteries had been in my open backpack while I was flying so now I keep my spare batteries tucked inside my jumper (body heat) to keep them nice and cosy.

My personal experience and recommendation. I took my drone up with me skiing and decided I will take a few turns first before going back to my car to film. So I left the drone in the car while I was on the slopes. When I got back and tried flying it it started giving me a critical battery error and started forcing itself to land. After talking to other people in this forum, I made the mistake of flying with a cold battery. It was about 0F outside. What I have done now is to bring the battery with me while I ski inside pocket of my innermost layer to keep it warm. I have never had issues after doing this and I have flown in -12F before. Keep in mind though that you are operating outside the recommended range and if you crash DJI is not going to warranty it.

Hello, please refer to the screenshot and remember to warm up the battery before your flight.