The person who swears in the president of the United States is the chief justice of the Supreme Court.
When Donald Trump was sworn in as president he was sworn in by Chief Justice John Roberts.
Chief Justice John Roberts swore in Donald Trump on the steps of the Capitol Building.
So whoever is Chief Justice of the Supreme Court at the time the new presidents get sworn in that person will be the one to have the President raise their hand and get sworn in as President.
The President takes an oath with their hand on a bible and they are supposed to protect that oath they were sworn in on although not all presidents will do what they say they will do even though they took the oath.
However some presidents have sworn on a book of law and not the bible.