The physics
In a metal, charges move freely. They stop only when the field in the metal is zero. So extra charge sits on the surface.
in the metal: E = 0 just outside: E = σ / ε0
A charged shell gives no field inside it. Outside, it acts like a point charge at its centre.
E = kQ / r² outside V = kQ / r outside, kQ / R inside
V adds as plain numbers. Each shell adds kq over the larger of r and its own radius.
A charge in the hollow pulls an equal, opposite charge onto the inner wall. The outer wall takes the rest, spread evenly.
An earth wire holds a metal at V = 0. It does not make its charge zero.