This is made from just one mesh plane and one solidify modifier. The different heights come from using different weights as you assign vertices to the group. You could probably assign the weights with weight painting, which would be fun. Yet to try that.
Tried using two or more solidify modifiers but that definitely doesn’t work. The other way was to use a vertex group and the factor slider in the modifier panel. This only gives two heights.
Here is the weight map.