What you want is a transistor, either a mosfet or a BJT.
Transistors are subtle when you try to use them as amplifiers, but when you use them as switches they're fairly simple. When the control pin's voltage is at the OFF level, the resistance between the other two pins is huge.. usually more than 20 megohms. When the control pin's voltage is at the ON level, the resistance between the other two pins is low.. a few ohms for a BJT or a small-signal mosfet, a few thousandths of an ohm for a power mosfet.
How is the switch connected to the circuit you want to control?
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