I am very new to Blender without any 3D design background. I am designing a solid object as shown in Figure 1 and Figure 2. However, when I export the object to STL file and view it by Preview in Mac OS, the object appears to have an unwanted hollow (Figure 3). Another object that I design in the same project has no problem Therefore, could you please help me to solve this problem? Thank you very much.
I guess your Mac Viewer shows only faces, oriented to camera, while Blender (by default) shows both sides
For viewing what is the back side and what is front, you should turn face orientation overlay
You can recalculate face orientations using Shift+N, but your object should be manifold.
You can also use Alt+N to flip orientation face by face