I've seen many questions with the mark [closed] in the title. Does it mean that they will be erased later? Or just remains as closed?
They may be deleted later. It depends on the close reason and what attention they receive (some questions can just go unnoticed). Duplicates which help point the way to other questions (that is, they are worded differently and aid searching) should not be deleted, but many others should.
Users get access to delete and undelete votes at 10k reputation. Moderators (with diamonds) can always delete.