When I see a put on hold message, it will always show put on hold by someone. I would like to know who these people are. Are they moderators?
Not necessarily mods
The users of Physics.SE who have over 3000 reputation obtain the privilege to "Vote to Close" a Question. At 5 close votes, the post is automatically "Put on Hold". After a week, its status is changed to "Closed", unless it is reopened in the meantime. This is also mentioned in our Help Center.
Moreover, users which have a gold tag badge in either of the tags of the question can single-handedly close it as a duplicate (aka "hammer"). Moderators can single-handedly close / reopen any question.
Note that you can distinguish moderators from all other users, because they have a diamond (♦) after their username.