Article I, Section 1 of the US Constitution says: “All legislative Powers herein granted shall be vested in a Congress of the United States, which shall consist of a Senate and House of Representatives.” One way to paraphrase that is: Federal law is made by Congress.
In the modern US, this is not quite true. Some federal law is made by Congress, but quite a lot of federal law is made by government departments and administrative agencies. In many cases, Congress passes fairly general laws, and those laws instruct the relevant agency to write rules implementing the laws, and then the agencies write more specific rules. Sometimes these rules just fill in details in a comprehensive statutory scheme. Other times the agencies have pretty broad mandates to write rules that are in the public interest, and they get to set their own agendas and decide what that means.