The Supreme Court stands just across the road from the Capitol Building (Between 1st Street NE; East Capitol Street; and Maryland Avenue). It was only established there in the twentieth century - previously the court met inside the Capitol Building itself.
Under the doctrine of 'Separation of Power', the one person cannot be in more than one of the three branches of government (legislative; executive; judiciary). The strict application of the doctrine requires separate personnel, but the branches must work together for government to function. The President nominates justices; the Senate confirms - but once appointed, members of the Supreme Court cannot be removed, save by the extraordinary process of impeachment.
There is a Chief Justice (John Roberts), who sits with eight Associate Justices. They are appointed for life, but may retire or resign (or be impeached!).
Sometimes the Supreme Court is referred to as SCOTUS (Supreme Court of the United States). Since the leading case of Marbury v. Madison, the Court has been able to strike down all acts of the other branches where these are judged to be unconstitutional - including legislation.
By contrast the Supreme Court in the UK, which will open in the Middlesex Guildhall next year (also across the street from the legislature (Parliament)), will not be able to strike down legislation.