Remap the §± key on my magic keyboard
This will require your participation :)


Download and run free Key Codes utility.
Press ± § key on your keyboard and write its HEX code from the utility screen.
Press ` _ key and write HEX code for it.
Open Terminal app and enter the following command (copy and paste it):

hidutil property --set '{"UserKeyMapping":[{"HIDKeyboardModifierMappingSrc":0x700000032,"HIDKeyboardModifierMappingDst":0x700000035},{"HIDKeyboardModifierMappingSrc":0x700000035,"HIDKeyboardModifierMappingDst":0x700000032}]}'

HIDKeyboardModifierMappingSrc is what you press on your keyboard and HIDKeyboardModifierMappingDstKey is what will be the result.
Pay attention to the bolded numbers endings!! You will need to replace them with actual readings from the Key Codes utility (Step 2 and 3). I wrote most probable values, but they may differ in your case.


As an alternative solution, you may want to try Flamingo app.

Reference to the hidutil usage is here: Technical Note TN2450.