My understanding:
An inertial unit alone can calculate drift angle, since it knows the aircraft heading & the actual track across the ground. Difference between the two is your drift angle.
Similar thing for airspeed wit the addition of an Air Data Computer - you have probes / computer for calculating airspeed as you fly through the air, and an inertial unit which is calculating your speed across the ground. Difference gives you the windspeed component.
Olden days (C130H for example) they used things like doppler radar for this, but I'm not sure that's still in use now.