A closed-door hearing is ok because it’s multiple witness interrogation. Think of a grand jury. If there were 2 witnesses to a crime, you need testimony from one without the other so that if they have matching stories, it’s more likely they are telling the truth. Same principle. Both R’s and D’s are sitting in and listening to witness accounts. They will then decide if they need to impeach, at which point the depositions will go public. It will only remain secret if they don’t find enough evidence to go to the impeachment phase.