Non-Manifold Geometry

 Non-manifold geometry is defined as any edge shared by more than two faces. This can occur when a face or edge is extruded but not moved, which results in two identical edges directly on top of one another.

Through that I locate the problem vertices/faces and then fix them. Sometimes there's a reversed face welded to a regular face. Sometimes there's double faces. Sometimes it's just a bloody mystery so deleting and reconstructing segments through bridge/fill solves the problem.
https://www.reddit.com/r/Maya/comments/aegm06/help_non_manifold_vertex/

Generally beveling every face to see which ones can and which ones can’t help you figure out the problem. Continue going around either hiding (if it can bevel) or deleting (if it can’t bevel) or moving individual faces around to see what doesn't move correctly. Manually fix faces by either bridging, filling, or extrude/target welding.