“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.