Step 1 (i)(inside): For a Gaussian sphere of radius $r<R$ inside the shell, enclosed charge $=0$ (all charge resides on the shell's surface), so by Gauss's law, $E=0$ inside.

Step 2 (i)(outside): For $r>R$, enclosed charge $=Q$: $E\times4\pi r^2 = Q/\varepsilon_0 \Rightarrow E=\dfrac{Q}{4\pi\varepsilon_0r^2} = \dfrac{kQ}{r^2}$ (same as a point charge at the centre).

Step 3 (ii): For an isolated non-conducting (thin) sheet of charge density $\sigma$: field on each side $=\dfrac{\sigma}{2\varepsilon_0}$ (field emerges from both faces of the sheet, splitting the flux).

Step 4: For a charged CONDUCTOR surface, charge resides only on the outer surface, and just outside a conductor's surface, the field is $E=\dfrac{\sigma}{\varepsilon_0}$ (using a Gaussian pillbox with zero field inside the conductor, so all flux emerges through the outer face only).

Step 5: Comparing: $\dfrac{\sigma}{\varepsilon_0}$ (conductor) is TWICE $\dfrac{\sigma}{2\varepsilon_0}$ (non-conducting sheet), for the same $\sigma$.