I would guess most people who listen to music while they work have the volume way too far up for that amount of exposure. Anyone on an iPhone should go into Sounds -> Headphone Safety and reduce loud audio to 75 dB. You'll probably be shocked by how quiet that is.
It's as simple as you'd expect: AirPods and beats have their SPLs hardcoded, everything else is estimated based on volume control.
Although Apple says there can be bad interactions between this and live listen, so maybe there is some realtime monitoring happening.
https://support.apple.com/guide/iphone/check-your-headphone-...
I wonder how they determine that? Especially with Bluetooth, you have no idea how much sound energy is being put out, and can't infer it from power draw or anything similar.