Responsibilities of Congress under the separation of powers:
US Federal Government is designed in such a way that it is split into three branches namely, legislative, judicial and executive with a purpose to prevent one branch gaining more power than the others. This is called checks and balances.
Congress has its exclusive power to legislate and make laws. For example, Congress has solo authority to raise and decide how much money to be spent. However, recently there was a apprehension whether President has discretion not to spend money allotted by Congress.