How can the Community user close questions? I know it performs various tasks, but this is the first time I've seen this particular action.
As reported in New UI encourages askers to confirm or dispute duplicate votes, users who asked a question that received at least one closing vote now see a banner asking them if the other question helps them.
In the case the OP clicks on That solved my problem! the question is automatically closed, after a confirmation dialog informing the user the question is going to be closed as duplicate is shown; the list of the users who closed the questions includes the Community user.
Hovering the close reason will show for which exact reason the question was closed by the Community user.