I wanted to know the physics behind the working of such a model. Specifically, the relationship between the force applied by the Rider and the weight of the passengers.
To accelerate, the peddler is accelerating the weight of three people plus the bicycle. Once moving, the rolling resistance generally scales with the weight of the total vehicle. the rear wheels and tires have to be sized to carry the weight. The Wikpedia article gives 0.0055 as a typical rolling resistance coefficient for typical BMX tires, which is more like what I would expect than road race tires for a vehicle like this. Vehicles like this have rather low gearing so the pedal force is within the strength of the rider. For a given vehicle, the pedal force will be proportional to the total weight carried. The speeds are low enough that air resistance is not a huge issue, though the sun cover increases it.