Why am I getting different shading on different faces when I merge the vertices? I started with a bowl shaped geometry with no top and no bottom.

I wanted to add geometry to the top of this semi-sphere, so that I can use sculpting to mould it to a desired state. Does Sculpting create any new Geometry?.



On bottom side I followed How can I add vertices to intersection of two edges? to add geometry and on the top side I extruded the vertices and merged them to the centre.
It may be that you have inverted normals. In order to 'repair' the object, follow those steps:


Enter Edit Mode by pressing Tab.
Select every vertex in the model. You can press 1, then a (maybe even times).
Press Shift+N to recalculate normals outside, or Shift+Ctrl+N to recalculate inside.
Exit from Edit Mode by pressing Tab.


Those steps should help, but if you have messed up something other than normals, it may not help. You can also enable "Face orientation" in overlays menu in 3d Viewport to see blue/red faces indicating normals.