We will start at the very beginning.
As seen in the attached picture, within Cinema 4D, and created solely for the sake of this discussion, I create a simple object, and unwrap it.
Next, in my personal pipeline, I will "Create UV Mesh Layer", and save the Texture out as a .PSD.
Next, I open the .PSD within Photoshop to apply my Textures to the UV Layer.
Herein begins my question.
Simply creating a few new layers and dragging, and stretching textures into place where they belong on the UV, is not the problem. My lack of understanding is as follows.
Ultimately, this model, and its UV, are going to end up inside Unity. I need to understand how to apply a number of multiple Texture Maps, (ie Diffuse, Height, AO, Normal), inside of Photoshop, while stretching, and aligning them to the UV.
One single Texture is no problem, drag it, get it into shape etc, no problem, but let's say for every face of my simple object I need/want to not only have the Diffuse, but the Normal also. Both maps would have to be, "stuck" together throughout the placing, stretching, and resizing phase, or they will end up out of sync with one another.
So that again, ultimately, when I drag the finished UV, to my object within Unity, the object will automatically be UV mapped, with all of the Texture Maps correctly aligned.
I fully understand that Unity has it's own way of adding Texture Maps to objects, however, that is only going to work if you are dealing with an object that did not need to be UV'ed, flat objects such as a terrain, or a plane, or a Cube, but that will not work with a single object that needed to be UV'ed in the first place, such as my object, in which I purposefully raised the center of the top up, just so it wouldn't be a simple Cube.
One may say that this question is better suited for a Photoshop forum, but I don't agree, given the approach of my question. Within a forum of this caliber, CG related scenarios are going to be more in sync with what I am attempting to do, as what I am attempting to do is almost a mainstream procedure in UV mapping, versus something that someone would/could help me with within a Bitmap Graphics based forum.
I can only hope that I am explaining my scenario correctly, thank you for your time.
Usually is very important to paint into the object all PBR textures you need into Unity or UE for creating stunning effects on the surface, as Albedo, Roughness, Glossy, Reflections map, Height map, normal map, and more (I don't remember all unity channels). For best results usually you need an instrument that give you the possibility to generate or paint maps directly to the objects. For example: usually when you need to work on some rocks or robot plates os similar objects, you need a normal map for creating bumps on the surface, so if you sculpt into the object some parts, you can convert and bake it to a simple normal texture with bake command.
If you need some other painting parts you can use tools as BodyPaint to create other maps by manually painting on the same parts of the models. In this way you can sculpt the object and also you can paint on sculpted surfaces for create glossiness maps, albedo maps with coloured parts and so on.
Base on some projects we done with Unity, I learned that you must use tool as BodyPaint to create, patch, paint, all channel that you will use on Unity as Albedo, Normals, Reflections map, Luma maps, and much more.
The best way is to learn how all maps on PBR materials works, to understand what is Albedo, what is height map, what is normal map, how to make glossy mask into the reflaction channel and more.
What kind of experience you have with PBR Unity channels? if you know how it works you can use BodyPaint to start to create channels and paint into it.
BodyPaint give you many channels for painting and you can achieve good results...