Try something simple: create a sphere, convert it to polygons with Mesh / Conversion / Make Editable and select the Points tool. Select the Rectangle Selection tool and, in the Front view, select the right half of the Sphere object's points and delete them. Create a Symmetry object and, in the Object Manager, make the sphere a child of the Symmetry object.

The right half of the sphere is generated and the sphere is complete once more. Note that only the left-hand side has points. Now edit some of the points on the left-hand side, perhaps with the Magnet tool. Notice how the generated side is updated automatically.