To use Extrude tool inside modeler
Multiply > Modeler > Extrude or press shift (e)
If you would like to use the Extrude tool, first you need to go to the Modeler option from the Multiply menu. After that, you need to choose the Extrude or press the shift (e) to select Extrude tool. It is used to extend the model thickness. Normally when we import a 2D format of drawing into modeler, it will appear to be a flat object. That is why you need to extend it to become a 3D object, which is the function of Extrude tool.
You could see that the object has changed from a flat object to a 3D object