The how is actually just further hypotheses. It's turtles all the way down:
There is a car. We think it drives by burning petrol somehow.
How do we test this? We take petrol away and it stops driving.
Ok, so we know it has something to do with petrol. How does it burning the petrol make it drive?
We think it is caused by the burned petrol pushing the cylinders, which are attached to the wheels through some gearing. How do we test it? Take away the gearing and see if it drives.
Anyway, this never ends. You can keep asking questions, and as long as the hypothesis is something you can test, you are doing science.
In the vein of "can a biologist fix a radio" and "can a neuroscientist understand a microprocessor", see https://review.ucsc.edu/spring04/bio-debate.html which is an absolutely wonderful explanation of how geneticists and biochemists would go about reverse-engineering cars.
The best part is where the geneticist ties the arms of all the suit-wearing employees and it has no functional effect on the car.
There is a car. We think it drives by burning petrol somehow.
How do we test this? We take petrol away and it stops driving.
Ok, so we know it has something to do with petrol. How does it burning the petrol make it drive?
We think it is caused by the burned petrol pushing the cylinders, which are attached to the wheels through some gearing. How do we test it? Take away the gearing and see if it drives.
Anyway, this never ends. You can keep asking questions, and as long as the hypothesis is something you can test, you are doing science.