“What mechanism would even support doing that?”
Your phone, if you have wifi or Bluetooth turned on, is constantly broadcasting to any other phone or device nearby that has wifi or Bluetooth enabled. These broadcasts contain a unique identifier that could be used to track your phone (Bluetooth ID, or MAC address), even if you have tracking turned off on your own phone. I think Apple fixed this issue with more modern iphones by randomizing the MAC address every so often, but older iphones and Androids may still be vulnerable, and the Bluetooth ID is still unique anyway, so that’s another vector that could be used.
Do you have any evidence that a particular app in widespread use is actually collecting any of that data? There’s no reason to have Bluetooth turned on unless you’re actively using it. Really, same goes for WiFi. Just another battery hog. But assuming they’re on, who’s actually listening and using it for anything?
Again, typically when some bad actor comes out and collects that kind of data, privacy advocates find out pretty fast (by monitoring all incoming/outgoing communications on their devices) and go nuts about it.