Photoshop 3D Help

Each of those are materials,  so already consist of a number of 2D texture layers. To access them click on one of the materials and then in properties click on the folder next to the texture (base color, metallic , roughness etc)  you want to edit and choose edit texture (shown here for base color). A psb file will open with the texture - you can edit it the same as any other 2D image and when finished close and choose save to update the material.

Dave