Silver plated will usually be copper based (like brass) or nickel based. Usually it is not a pure copper base, as copper is too weak. Pure nickel under silver plate is an older plating method and rarely used anymore. Nickel is magnetic, whereas silver, copper brass and cupronickel are not.
Nickel silver, Maillechort, German silver, Argentan, new silver, nickel brass, albata, alpacca, is a copper alloy with nickel and often zinc. The usual formulation is 60% copper, 20% nickel and 20% zinc.