Well if you are developing these models for a game then I would recommend you to texture each mesh/object separately and then join the meshes into single one and then bake the textures into a single atlas. 

Having a single texture/skin/atlas instead of multiple ones will reduce the number of draw calls required to render the object thus improving the game's performance.

5 hours ago, Poigahn said:

My question is this, Should I Texture/Skin each Item Separately or Join all Items together to form the Final Model, then UV Unwrap The complete model and Create the Texture / Skin.

For performance you should try to keep all parts as one mesh. However things like windows that need special materials are imported separately.

Normally a 3D artist would work on something like a wheel on it's own, UV map it etc. Then before exporting all the Opaque pieces are merged and there textures atlas together, while transparent or special shader parts are removed.

Some artist do texturing at the end, so atlasing isn't needed; instead they just merge all objects and UV map again.

The important part is to reduce the amount of materials. Two object using one material should be better than one object using two materials.

But it is only a rule of thumb. Material complexity, edge complexity, poly count and all kinds of other factors will also have a impact.

2 hours ago, Poigahn said:

 

  I am still learning about 3D modeling - So the Terms "Bake" and "Atlas" I do not understand.  I will do other research, If you could elaborate on the terminology That would Help!  

Once I figure out how my models would be used, I would like to give the glass or "Chromed" areas a reflective quality.  Are you saying that these areas need to be kept separated from the rest of the model ?

Essentially baking is a process of combining multiple materials/textures into a single one which can then beapplied to a single mesh/object.

Atlas is just another word for texture, but I guess it's mostly used in 2D games to refer to textures which have multiple sprites embedded into them. So I might have used the term incorrectly here.

Overall I would say that the modelling requirements ultimately depend on the 3d engine being used, and how it is being used. Guidelines for environmental art might be different than for vehicles, which might be different than for skinned meshes (moving characters).

Most 3d engines can cope reasonably with quite a variety of stuff being thrown at them but specific games may have particular art pipelines that like things to be done in a certain way .. programmers / technical directors will decide on this.

The main difference in texturing as you seem to have gathered is between either

1. selecting sub-materials for groups of faces, and uv mapping these separately, versus
2. uv unwrapping the whole (or most of) the mesh and painting this either in 2D (e.g. photoshop) or 3D (e.g. substance painter, mari).

Sometimes artists will use method 1, then bake the textures down into a uv unwrapped texture, and then apply method 2 to further refine.

This is an example of method 1. The same wooden material is used on each stake and UV mapped with overlapping UVs. This can either be used as is in the engine, or be baked down to a UV unwrapped texture (like an actor skin texture) so that each face has a unique space in the UV map, and other effects can be applied like ambient occlusion etc.

12 hours ago, Poigahn said:

Once I figure out how my models would be used, I would like to give the glass or "Chromed" areas a reflective quality.  Are you saying that these areas need to be kept separated from the rest of the model ?

No these are not separate because "Chromed" like effects are part of the Opaque rendering.

I quickly made this to demonstrate what a average material setup in a AAA game would look like. The gif lost some quality but you need to see it moving to see all the surface details.

This is a simple flat polygon, no depth or any details modeled on it.

To make this I used seven different texture maps, namely: Albedo, Depth, Normal, Emit, AO, Rough and metal maps.

The shader used here is Unreal's Opaque BPR metallic shader. It is designed to create +/-80% of all known surface materials.

If I wanted a special part, like glass I would have needed to use a transparent material. For skin I would have needed to use a subsurface shader.

Whenever you deal with a surface that isn't just Opaque, you will have to seperate that from the rest. For any Opaque surface you use texture maps to tell the shader what part is wood, metal, plastic, rubber, mirror, lights etc.

A Car model would mostly be Opaque, only the glass would be seperated. 2 materials.

A human model will have things like cloths using a Opaque shader, the skin would be a separate subsurface shader, eyes would have a transparent layer(only when near) and hair would need it's own shader to look fuzzy. 3-4 materials.

12 hours ago, Poigahn said:

"Atlas"

"Atlasing" is the term for uv mapping more than one object to the same texture. Then you would use texture maps to tell the shader what part should render how. It is very much like a sprite sheet but instead made from textures for 3D models.

Like if you had a table with a vase on it. It would be a waste to use more than one material, so you atlas them into one. here is a example from the web: https://www.creativeshrimp.com/game-level-texturing-texture-atlas-part-35.html

There is a lot to atlasing. Professionals who depend on Texel density, to sync with a team, depend on atlasing.

12 hours ago, Poigahn said:

"Bake"

It means to capture data in 3D.

The image above I "baked" all the texture maps, using a 3D model I made. Meaning I captured 3D data into a 2D textures. Like a normal map, is light direction data.

Using 3D meshes and some smart texture baking tricks, I created all the data I needed. This was taken to Substance Designer where I extracted extra data from my textures and created even more maps; ending up with seven maps total.

Sorry for making the post so image heavy, I needed to show what I was doing to explain. I hope this helps a bit.

7 hours ago, Poigahn said:

Any recommendations on 3D Engines and books on how to use those engines beyond that of the beginner media ?

Unreal is what I used above, it is the best 3D engine for now. Not beginner friendly but worth strugling with.

Unity is much more beginner friendly however it is a pain to use for graphics. Making small tweaks like the depth map here, will require merging of textures, and other tweaks will require editing the shaders.

Personally I find the tutorials on the net better for learning these engines than the books. Both Unity and Unreal have a set of official tutorials that teaches most of what is needed.

Unity: https://unity3d.com/learn/tutorials 

Unreal: https://www.unrealengine.com/en-US/video-tutorials and https://docs.unrealengine.com/en-us/Resources

As for 3D, there isn't really any good books on game art, instead look to this site: http://wiki.polycount.com/wiki/Polycount it is the Polycount forum wiki. Polycount is a site full of industry, hobby and beginner artists. A good place to start: http://wiki.polycount.com/wiki/Prop_Modeling

My advice is start simple and keep your art consistent.