/3/ - 3DCG

>>804536
flipping just flips the normals of every single face (duh), recalculating re-assigns them a direction and tries to make sure that adjacent faces share the same orientation.
If you for example mirror something by scaling it with -1, the normals of that part will be facing in the wrong direction. you can select these parts and flip them, or just select everything and recalculate with the same result.

"Recalculate Outside" is what usually works, but the important part is that the normal vectors point away from the interior of the mesh. If you toggle "face orientation" your mesh should be blue.