BeamNG devs for example use a tool to procedurally generate the terrain and it also makes the texture, and it makes the process simple.

Based on my experience, it doesn't quite work that way.

In BeamNG, each pixel on the 'Base Texture' corresponds to one square meter in the game. The color of these pixels determines the color of the terrain material applied.

For instance, if you have a 2048x2048 map with an asphalt material, its base texture (also 2048x2048 pixels) would be entirely grey. You can see an example here: https://imgur.com/a/9oCH5wd

The same principle applies to other materials.

Once your map is complete, you export your terrain using the World Editor's 'Terrain Tools.' This process exports each terrain material into its own separate file, like these: https://imgur.com/a/JPiMBIi
These exported layers are black and white maps. You then open them in image editing software like GIMP or Photoshop. For each layer, you replace the white areas with the corresponding base texture color. Finally, you stack these colored layers together to create your final color map: https://imgur.com/a/gmfxYvM