> Almost all federal departments and agencies derive their authority from acts of Congress.
Partially, as we need to be precise in our language.
Congress can design an agency, determine what powers it has, determine agency procedures and fund agencies.
It's up to the President to execute the law within those boundaries, including selecting the head of that agency (with Senate approval), and determining how the agency executes the law which can be incredibly broad if Congress wasn't proscriptive.
Thus the authority to execute the laws is delegated to the President by the Constitution who then delegates that power to the head of the agency which acts on the Presidents behalf.
More broadly, the Supreme Court has made clear that the Constitution imposes important limits on Congress’s ability to influence or control the actions of officers once they are appointed. Likewise, it is widely believed that the President must retain a certain amount of independent discretion in selecting officers that Congress may not impede. These principles ensure that the President may fulfill his constitutional duty under Article II to take [c]are that the laws are faithfully executed.9
https://constitution.congress.gov/browse/essay/artII-S2-C2-3...