By GFX, do you mean 3D rendered designs, or 2D digital drawings? Your first, and fourth example are drawings, and everything else is 3D graphics. Anyways, if you want to make the latter, you first need the required software.
Minimal Software Requirement
– Cinema 4D / Blender. In my opinion, C4D is better for beginners. It also brings better results in terms of rendering, but that depends on an artist of course.
– Photoshop / Gimp / Paint Net. Out of these three, Photoshop is the most versatile. It’s also fairly easy to use, and very powerful. You use it for post-render phase to fix up a lightning and add details.
– Roblox Studio. Not much to explain here. You need it to export models.
– Substance Painter (Optional). This is a program that could help you add and/or rework the textures. It also has a lot of very good textures on Substance Source, but I am not sure if you need a separate subscription for it or not. You can always grab decent free textures from the web, though.
I have this. What should I do next?
– Go to Roblox studio, and put all the characters and models you want to use. Export them one by one into a file. In the end, you should have .obj model files, and .png texture files.
– Go to 3D editing software, and input a character rig.
– Change character texture to your desired one.
– Use the rig to position the character in desired position
– Put all the models and accessories on the character.
– Create environment around the scene. Use different models, and textures.
– Render the scene. Make sure it’s correct resolution. Also, it’s better to render it as alpha channel unless you already added sky enviornment in C4D or Blender.
– Input the file into Photoshop. Adjust brightness, and contrast to your desired levels.
– Work with curves, and adjust the coloring.
– Input background into the scene (unless it’s already done), and adjust it as well.
– Add details, such as scratches, dirt splashes, etc.
– Merge all layers, and adjust the brightness and contrast one last time.
– Save the file as .png or any other desired extension.
– You are done! Congratulations.