The harp doesn't actually lie flush on the soundboard; there should be some space in between. The harp is just there to bear the tension of all the strings. The strings are connected to the soundboard via a wooden so called bridge, which is a raised wooden element glued onto the soundboard. It has pins in it which the strings are held against in tension. Bridges have a coating of graphite usually, and combined with the fact that most notes have 3 strings, it creates kind of a blocky pattern.
Here you can see a soundboard, bridge, and hitch pins on the harp. The hitch pins on the harp can be seen up top, then in the middle the bridge and bridge pins, and then below that the soundboard. The hitch pins are located on the opposite side of the keys in a grand, or on the bottom of the piano in an upright. You can see on the right of the image that the harp has a structural element which floats over the bridge, with plenty clearance.
Here you can see a soundboard, bridge, and hitch pins on the harp. The hitch pins on the harp can be seen up top, then in the middle the bridge and bridge pins, and then below that the soundboard. The hitch pins are located on the opposite side of the keys in a grand, or on the bottom of the piano in an upright. You can see on the right of the image that the harp has a structural element which floats over the bridge, with plenty clearance.
https://www.chuppspianos.com/wp-content/uploads/2014/03/Stei...
P.S. In some pianos you can really hear a metallic undertone produced by the harp, I find Yamaha pianos have a distinct sound that has it.