Let's keep it simple.

The structural part of this question is done by the designer of the aircraft . . . engineers. Pilot's don't need to worry about this, but we do need to keep an aircraft within the limitations given (see Section 2 of any POH for limitations).

The weight and balance is what pilots and mechanics have to do. We need to keep from exceeding the CG envelope limits, zone limits and floor loading limits. That's about all.

For pilots to do W&B we must have the EW and EWCG for the aircraft. Then we add the weights for fuel, the pilot, passengers, cargo, equipment, etc. at the correct stations.