IIUC you need a higher temperature source from which to pump heat, so in the winter, where do you get that temperature differential?

Thankfully, you are wrong about that! Just like you don't need a cooler source _from which to source cold_ in the summer with AC. Technology Connections has a fun (well, to me) video on how heat pumps work and are great. Literally just AC units running in reverse, thanks to having a valve.

https://www.youtube.com/watch?v=7J52mDjZzto

Yes, Technology Connections is a great channel and added a couple of new videos about heat pumps in the past month.

https://www.youtube.com/c/TechnologyConnections/videos

At a certain temperature differential you need to source "heat" from elsewhere, which is why up north you either need a geothermal heat pump (underground piping) or you supplement with gas or electric heat.

Some very efficient pumps can work down to 0 degrees, but most start losing efficiency at 25 to 40 (Fahrenheit).

For some parts of the world, a heat pump is perfect - pump in heat in the winter and heat out in the summer, but when the outside temperature can hit -40º Fahrenheit (which is -40º Celsius), you're gonna need something else.

For the overwhelming majority of the world they are perfect. Even in high latitudes, they might only reach those extremes for a few days in a year, that's when you would require a backup.

Losing efficiency is fine since they are so ridiculously efficient to begin with.

This is not true, any more than your fridge needing a source of cold from which to cool itself down.

The temperature differential is between the outside temperature and -273°C. There's still heat energy to be captured even on a cold day.