https://knowledge.autodesk.com/support/maya-lt/learn-explore/caas/CloudHelp/cloudhelp/2020/ENU/MayaL... When you extrude and there are two things that can happen. First, two or more faces share a single vert but no edges. . The solution is to add more division or an edge loop. A bevel could help too but that depend on geometry.
Second, the uv's shells share a single uv and no edges. With the uv's if you do automatic uv's this will separate the shared uv's. Then just sew the pieces back together. If you try to use uv unfold and you get an error of non manifold geometry that means uv's shells share a single uv and no edges.
