BAKE BASIC USAGE
To export a textured model you have to bake its textures, in order to end up with a standard PBR Material.
Save the Project, select the Object, click on the Bake button.
Enable all the textures you want to bake, in this example Diffuse, Specular, Roughness, Normal. Set the desired Bake size.
Choose a path on your computer where to save the Baked textures. Click OK to start the baking process, or Esc to abort.
At the end of the process, next to the .blend file, you will find a new folder, containing all your baked textures and your model saved as Fbx, ready to import into another file or another software
In the Blend file a new scene is created, containing a copy of the model with a new simple PBR Material and a new UV Layout.