Essentially, an airline’s hard product is the plane itself, and the airline’s soft product is the service, food, and the drinks.
The real differentiation is that hard product is hard to alter (requires construction), while soft product can be changed in 5 min with a phone call. Accordingly, limo service is a soft product for the airline (and a hard product for the limo company, at least as far as the car goes). For airplanes, the actual cost of the hard product is the airplane’s downtime during fit out (often greater than the cost of the hardware being added).
A first/business class hard product is anything physically attached to the plane, which doesn’t differ from flight to flight. For example, the seat, onboard amenities (shower, bar, etc.), size of the entertainment screen, etc.
A first/business class soft product is anything which can differ from flight to flight. For example, food, drinks, service, amenity kits, etc.