This metal frame inside the piano is known as the “plate.” Many people erroneously refer to it as the soundboard. The soundboard is in fact just that – a thin board behind the plate that resonates with the strings and amplifies the sound. The soundboard is less than half an inch thick, weighs only a few pounds, and is purely acoustic. The plate weighs anywhere from 150-500 pounds depending on the size of the piano, and withstands tens of thousands of pounds of string tension at all times throughout the life of the piano. It is the structural core of every acoustic piano.
And sorry, even though it’s often a nice shiny gold color, the plate is not made of gold! Or even brass, bronze or copper. Other than a few rare, experimental exceptions, all piano plates since the 1800s have been made out of cast iron.
And that’s why pianos are so heavy! You should get them moved professionally unless you know what you’re doing and have proper equipment.