The Senators and Representatives shall receive a Compensation for their Services, to be ascertained by Law, and paid out of the Treasury of the United States. They shall in all Cases, except Treason, Felony and Breach of the Peace, be privileged from Arrest during their Attendance at the Session of their respective Houses, and in going to and returning from the same; and for any Speech or Debate in either House, they shall not be questioned in any other Place.
Congress gets paid, and that compensation is determined by law. Congress has to decide on it, then the President has to sign it into law. If they have not committed a felony, treason or are not actively being violent, they cannot be arrested while in attendance in Congress, and they’re immune from criminal prosecution for the things they do and say as a legislator.
No Senator or Representative shall, during the Time for which he was elected, be appointed to any civil Office under the Authority of the United States, which shall have been created, or the Emoluments whereof shall have been encreased during such time; and no Person holding any Office under the United States, shall be a Member of either House during his Continuance in Office.
You cannot hold another Federal office while you serve in Congress.