This is a scheme of how use thevenin's theorem to reduce a complex circuit to a simply circuit.
1: Complex circuit with any load.
2: Disconnect the load, and calculate voltage between its terminals.
3: Thevenin equivalent voltage.
4.1: Short circuit fonts and calculate resistance between the same terminals from step 2.
4.2: Equivalent Thevenin resistance.
5: Thevenin equivalent circuit without load.
6: Put again the load, and notice: there's same current and voltage from the complex circuit.
It's valid to any load you choose, not only resistors.
