“I made a torus primitive and want to detach a section of it. When I select splines or quads and detach as element or copy, the detached part won’t select by itself; the whole torus is still selected. How can I make the detached section a completely independent object?”
You mention both splines and torus primitive. Splines and the menu that torus’ come from (primitives) are not the same, so I can’t be sure which you mean. Maybe you mean both? You mention editable spline, which only splines can be converted to, yet you also mention “quads”, which may mean a polygon (mostly 4 sides) in an editable mesh/poly, which is really just 2 faces. Forgive me, therefore, if the following has no application to your project. Feel free to fill in the blanks.
If you made a torus from a spline, and wanted to detach a portion of it into a separate object:
1. Make a spline. There is no torus spline, but a circle (a valid form of torus) will do.
2. Right-click, convert to editable spline.
3. Sub-select segment (hotkey 2).
4. Drag across the part you want to detach.
5. Under Geometry, click “Detach”.
6. You will be prompted for a name to call the new shape.
7. Select a name, and click OK. The original spline is still selected, but the segment you chose to detach isn’t any more.
8. Exit sub-object mode and select the new object you made.
9. Select the Adjust pivot point now if you need to.
Shapes (3d primitives)
1. Create a torus primitive (Create -> Standard Primitives -> Torus.
2. Right click and select editable poly/mesh.
3. Sub-select face or polygon (3 or 4). I couldn’t be sure which you needed from your email.
4. Drag to select in any window a part of the entire shape. It turns read.
5. Under “Edit Geometry”, select “Detach”. You have more options here than with splines, but the same idea applies. Give the new object you are creating by detaching a name.
6. Just like above with splines, your previous object is selected, but not the new one.
7. Exit sub-object mode and select the new object you made.
8. Adjust the pivot now if you need to.
Could it be that you did everything right, but didn’t exit sub-object/spline edit mode? That would prevent you from continuing, to be sure.