As @moonboots has said, many faces are inverted. You can see it because they are red but they should be blue. Enable Face Orientation under Overlays to see this. You also have this bug in your mesh which cannot be solved without technical understanding of the bug where 2 faces share the same sets of vertices.
To workaround this bug and solve this, first go to your Python Console and execute this command:
for m in D.meshes: m.validate()
Then you can tab into Edit Mode and press A to select all and press Shift+N to Recalculate Outside the Normals of the mesh (Mesh > Normals > Recalculate Outside
