> I understand that transistors amplify current but how do they amplify voltage

Any voltage through a resistor will produce a current, and any current through a resistor will produce a voltage. By properly connecting resistors at the transistor base and collector, one can turn a driving voltage into a current and the collector current back into an output voltage. The basic common emitter transistor amplifier circuit is a good example as it shows how a current amplifier like a transistor is used to amplify a voltage. Resistors are the secret that allow all permutations: voltage to voltage, current to voltage, voltage to current, current to current.