You can tell because the text of the Sixth Amendment starts "In all criminal prosecutions..."
Republicans are complete shit about the Constitution these days, aren't they?
Even that portion of the process isn't a criminal prosecution. The Constitution explicitly states that criminal prosecution could *follow* removal.
(1) You can't impeach a duly-elected president.
(2) Testimony is not evidence.
(3) Hearsay is not evidence.
(4) The Sixth Amendment applies to drawing up articles of impeachment.
(5) The president cannot be investigated.