[BLENDER] Issues with retaining concave meshes + Inquiry about 'smooth grouping'

Background Info:
I have recently picked up Blender in a bid to teach myself how to edit props to my liking. While I am very much a beginner, all has been smooth sailing up until now – creating and retaining concave collision models has proven to difficult and tedious to make happen. I’ve performed plenty of self-help searches here and elsewhere, but more often than not information is highly fragmented from one source to the next. Based on everything that I’ve watched and read through, the following is what I understand in a nutshell (whether on the dot, inaccurate, or completely off):

So yeah, if there’s one thing I’m still scratching my head about it’s comprehending the concept of group smoothing. I know there are ‘Smooth’ and ‘Flat’ shading preferences under (1) Object Mode > Tools > Edit > Shading and (2) Edit Mode > Shading/UVs (for faces, vertices, and edges), but I do not know whether there’s one way to go about creating ‘groupings’ or if the desired method of smooth grouping follows a case-by-case sort of basis.

Issue at Hand:
Several whacks have been attempted at completely hexing the ‘gutted’ version of the locker prop from the HL2 Leak content pack.

Upon being decompiled, the model’s concave mesh is destroyed whether or not it is imported to Blender. In hopes of averting the issue I’ve gone as far as to completely erase the existing mesh and creating a new one in its place. It looks like a really simple task, but from what I’ve attempted so far nothing has seemed to work. Smooth shading has also been applied numerous times, but that doesn’t help either. ‘$concave’ is already in the .QC file, too.

All and all, it’s gradually proving to be increasingly frustrating (and confusing). Any clear advice/guidance on how to knock out this turkey would be immensely appreciated. I will be happy to provide additional information/clarification wherever it’s needed.

Thanks for your time.

You would mark edges as sharp or soft in edit mode on the 3rd tab (same one as for marking UV islands). That, or you could use the edge split modifier.

I will certainly explore both options and see how they go. Thanks for the tip.

Once again I appreciate the help, but just so I understand: When marking edges as sharp/smooth, should every edge be taken into account (including those situated as concave corners or generated from knife splitting operations, for example)?