I’m gonna address these in steps if that’s alright.
- Shaders != textures, first and foremost. Traditional textures are just image files.
- There’s no battle between Designer and the Material Editor as they are vastly different. You use these together. Substance Designer lets you create procedural textures and then integrate them with the material editor and blueprints inside of UE.
- The material editor lets you create shaders/materials, though you can create actual baked textures by combining the material editor/blueprints (they aren’t gonna win any awards for being photoreal, though).
- Yes, you can still use Photoshop, and yes, you can combine those textures with Substance & the material editor.
- Textures are pretty software agnostic for the most part. They’re always some form of bitmap/image file in the end, so it doesn’t really matter if you use Gimp/Photoshop/Substance to produce textures (though Substance is inherently the strongest when it comes to dev work, IMO).
- Shaders are basically code files that aid with rendering scenes (characters, objects, etc) and are very different from textures.
Look into Photoshop/Substance Designer/Substance Painter for textures, and C++/HLSL/GLSL for actual shader coding. Also, take a look at Shadertoy on the web. You don’t need to learn C++/HLSL/GLSL to use the material editor in UE, but it would help you gain a fundamental understanding of what shaders are and how they work before digging into UE, though that’s up to you.