Geometry Tools - Stitch Faces

You can sew independent faces into a shell using Stitch Faces. If the shell is closed which has no open bound, you can also make it into a solid. To seperate faces from a shell or solid shape, you can use Decompose. In Decompose, you can also decompose non-manifold solid into several manifold solids.

To open Stitch Faces:

To stitch faces, check Stitch:

a. Raw independent faces
b. Shell after stitching

To decompose solid, check Decompose: