About the speccoc texture, how to make it. I'm quoting from a site for creating mods for GTA 5. Maybe the colors and the initial result will differ from the GTA specular map, but the LOGIC of creation is the same.
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Specular maps ("spec") are used to set how shiny or reflective a part is supposed to be. Typically, specular textures are black and white, dark parts have very little specularity, light parts more. GTA V also supports colored specular maps for specific materials, allowing for more precise adjustments.
The specific effect of specular maps depends on the shader used. For most materials, more red increases the shine of an object, while green makes a part more reflective. Blue seems to increase the overall specularity. For mesh materials, reflective plastic or metal parts tend to have a medium amount of red (usually around 60 to 120) and a medium to high amount of green and blue (often between 120 and 250). Matte plastic or rubber objects usually have a higher amount of red (up to 120, as more red would make the diffuse texture look brighter) and a low amount of green and blue (often between 50 and 100). For chrome parts, full white or very light grey is usually used together with a black or dark grey diffuse texture.
Creating specular maps
The process of creating specular maps often includes a lot of trial-and-error to test different colors in-game and see if it looks as intended. There are no generators for specular maps, they are typically created by adjusting the colors of the diffuse texture with an image editing tool.
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I’ll add on my own, thanks to this texture, you can visually “separate” the inscriptions on the car from the main color, as well as give the texture a mirror finish or, conversely, matte. That is, the texture becomes an order of magnitude better.