There are three main reasons that Congress is supreme. First, Congress can get itself involved in the actions of the other branches. It can override presidential vetoes. It can deny appointments to the executive and judicial branches. It can impeach officers of the executive and judicial branches. It can set legislative and judicial pay. It also has wide discretion in determining the size and shape of the executive and judicial branches. Every executive department and indeed every officer except the president and vice president are creations of Congress. And Congress also has total authority to design the court system as it sees fit. While there are aspects of foreign affairs it cannot attend to, it can basically govern the domestic affairs of the country by itself. Indeed, it actually did this during the period 1867–69, when Radical Republicans in Congress acquired a veto-proof majority over President Andrew Johnson. The main domestic power that Congress cannot exercise by itself is amending the Constitution — for that, it needs the assent of the state legislatures.
Second, the other branches are largely incapable of interfering with Congress. Members of Congress are immune from arrest when they are conducting legislative business. They set their own pay. Each chamber determines its own rules. Each branch also decides who does and who does not get to sit as a member. And no member is allowed to take a job in the executive department while serving in Congress. The Founders were so serious about this that they established a capital city apart from the states, and under the jurisdiction of Congress, to make sure that some local potentate could not use geography to boss the legislature around.
https://www.nationalreview.com/2019/01/congress-supreme-branch-of-government-not-coequal/
Second, the other branches are largely incapable of interfering with Congress. Members of Congress are immune from arrest when they are conducting legislative business. They set their own pay. Each chamber determines its own rules. Each branch also decides who does and who does not get to sit as a member. And no member is allowed to take a job in the executive department while serving in Congress. The Founders were so serious about this that they established a capital city apart from the states, and under the jurisdiction of Congress, to make sure that some local potentate could not use geography to boss the legislature around.
https://www.nationalreview.com/2019/01/congress-supreme-branch-of-government-not-coequal/