Hi Pilou,

re:
> Why the subdivision surface still stay in 6 separate faces and not redone a normal sphere when join or union?

The subdivision surface sphere is not quite exactly spherical.

Since it is not an exact match for a sphere there isn't any mechanism to try and convert it into an exact sphere.

If you want an exact sphere, you can make one with the Draw solid > Sphere command, instead of using SubD conversion.

- Michael