With the shirt shape finalized, make your mesh watertight by "sealing" the holes at the neck, waist, and wrist. A watertight shape ensures that only the top visible surface of the mesh is exposed at any angle. If an asset is not watertight, it may expose backfaces, or single sided faces, that could affect the rendering of your asset and performance when equipped.
To make your mesh watertight:
Select your mesh and switch to Edit Mode.
Starting with any hole in the mesh, hold Alt and click the last edge on the mesh. The entire edge highlights.
Press E to extrude the mesh and click after a small length is added.
With the new edge still selected, right-click and select Merge Vertices > At Center.
Press G to grab the new vertex and reposition it within your clothing mesh.
Repeat steps 2-5 until all the interior-exposing holes of your mesh are closed.