Take-a-look helmet mounted. I have multiple bikes, but one helmet, so no adapting between bikes. These days I mainly ride a drop bar rando bike and MTB, but I learned to use a mirror riding a recumbent, where turning to look back is not an option, so I quickly gained skill and confidence scanning with the mirror.

As I get older, and traffic gets heavier, and I ride for utility, looking over or under my shoulder is both more difficult and not adequate. I need to scan 360, something you can't do with a handlebar mirror.