As has been said…
Faces have a front side (white) and a backside (bluish gray) and renderers tend to ignore back faces.
Your ability to see and work with model geometry greatly depends upon how it’s rendered onscreen.
The active Style (and any unsaved changes made to it) controls how the model is rendered onscreen.
Experienced modelers generally build the model in Monochrome Face Style with Edges displayed.
Hidden Geometry, Profiles and Endpoints are toggled on/off as needed to suit certain situations.
Viewing your model in Monochrome Face Style reveals the faces of the bag component are reversed.
However, a closer look reveals there’s still a flaw; one face in the mesh is not reversed.
Notice it’s impossible to select and reverse that individual face.
That is, unless you adjust the Style to display Hidden Geometry. View > Hidden Geometry
For your interest:
You’ll find a compendium of performance modeling tips and best practices at the SketchUp Sage Site