How to add several volumes (cube, cylinder or sphere) in the same object?
Right-click on the cube you created in the previous chapter and choose the menu OBJECT > EDIT MESH ...

then select the CUBE, CYLINDER or SPHERE mode and click on the floor, even several times. To choose a new texture, click on the square.

Still with the EDIT MESH window open, pull items from your inventory into the world, they will also be added to your object.

To pull/stretch these volumes, select the MOVE/STRETCH mode, click on the point of a volume and arrows appear. You can choose :