Deep in the bowels of the Capitol there’s a tiny room with 4 or 5 Holding Cells that is the Capitol Jail. It’s not much because it isn’t intended to be occupied very long. The express purpose is to encourage the co-operation of Witnesses who are found in Contempt of Congress.
That process is very simple, the Chairman of the Committee finds the person in Contempt, there is or is not a vote of the Committee (it’s pro forma anyway) and the Sergeant At Arms of the House or the Senate or a member of his handful of staff (all duly deputized Police Officers) come to the Hearing Room and escort them to jail.
Should it become necessary they can call on the resources of the Capitol Hill Police who report directly to Congress and not at all to the Department of Justice.
In normal circumstances the Committee will send a request to the DoJ who will handle the arrest as a matter of course, but should they be unwilling to act Congress is fully empowered to do it themselves. Are the Capitol Hill Police limited by the actual Capital boundary where someone can step off the grounds and be out of their jurisdiction? No. The Capitol Hill Police can arrest you anywhere in the United States and all it’s territories.
Congress is not a toothless Tiger. As the primary institution of Government, the Article One power, their authority is nearly absolute under the Constitution. That they have chosen to abdicate it to the Executive represents a craven cowardice that is a tragedy of our time.