Which reference electrode is used to measure the electrode potential of other electrodes?

Calculate $\Delta G^{\circ}$ for the cell: $Sn_{(s)} | Sn^{2+}_{(1M)} || Ag^{+}_{(1M)} | Ag_{(s)}$ at $25^{\circ} C$ given that $E^{\circ}_{cell} = 0.90 \ V$. (in $kJ$)

The $EMF$ of a cell whose half-cell reactions are given below is .......... $V$.
$Mg^{2+} + 2e^- \to Mg_{(s)}; E^o = -2.37 \ V$
$Cu^{2+} + 2e^- \to Cu_{(s)}; E^o = +0.34 \ V$ (in $V$)

The standard reduction potentials (in $V$) of a few metal ion/metal electrodes are given below: $Cr^{3+}/Cr = -0.74$; $Cu^{2+}/Cu = +0.34$; $Pb^{2+}/Pb = -0.13$; $Ag^{+}/Ag = +0.8$. The reducing strength of the metals follows the order:

Can the absolute electrode potential of an electrode be measured?

What is cell potential and $emf$? Explain with suitable examples.