As @[moonboots][1] 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][2] in your mesh which cannot be solved [without technical understanding of the bug][3] where 2 faces share the same sets of vertices.

[![enter image description here][4]][4]

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 <kbd>tab</kbd> into *Edit Mode* and press <kbd>A</kbd> to select all and press <kbd>Shift</kbd>+<kbd>N</kbd> to *Recalculate Outside* the Normals of the mesh (*Mesh > Normals > Recalculate Outside*

[![enter image description here][5]][5]


  [1]: https://blender.stackexchange.com/users/34531/moonboots
  [2]: https://blender.stackexchange.com/questions/282322/is-having-2-or-more-faces-with-all-shared-vertices-a-bug-or-valid-geometry
  [3]: https://projects.blender.org/blender/blender/issues/70977
  [4]: https://i.sstatic.net/TbOaT.png
  [5]: https://i.sstatic.net/f566s.gif