To apply textures or materials in most 3d engines a 3d model will need a UV map. If you cant add a texture or material to an imported 3d model, it is most likely missing a UV map. Most CAD softwares do not have these features, as they generally aren’t intended for use in games and video. We will be using Blender, as it is free and has just about every 3d modeling feature you could ever want, rivaling paid options (despite it’s infamously bad UX and UI).

1. Import or open your 3d model in Blender
2. Go to “Edit Mode”. So we can view and select vertices.
3. Press the “A” key to select all vertices.
4. In the 2nd from the top toolbar select “UV”
5. Select “Smart UV Project”. This will automatically set the “Seams” for our UV map.
6. Click “OK”
7. In the top toolbar click “UV Editing”, so we can view our UV Map.
8. In the top right of that new window, click on the down facing arrow next to the symbol of two overlapping circles.
9. Check the “Show Stretch” box. This will display a heatmap over our UV, which denotes stretching.
10. If everything is blue there is little to no stretching and you are done.

If there is more stretching then you would like, you will need to procceed with manually editing your seams. Heres a good guide:

https://all3dp.com/2/blender-uv-mapping-simply-explained/