I'm not exactly sure how it works, but mobile OS accessibility layers provide support for "switch" devices which are used by disabled people. So I don't know if

I'm not exactly sure how it works, but mobile OS accessibility layers provide support for "switch" devices which are used by disabled people. So I don't know if this is limited to on/off switches, or variable knobs as well, but this may be the same sort of principle that would guide a car-based OS interface with physical controls on the dash.