The news was presented at the AAAS meeting in Phoenix, Arizona. Anna Fowler presented a synthesis of dozens of studies on near-death experiences and neuroelectrical activity around cardiac arrest. - https://particle.news/story/aaas-presentation-argues-consciousness-may-persist-minutes-to-hours-after-clinical-death
I can tell you from experience, you are not aware of being unconscious. It goes from the moment before you lose it, to when you regain without any period between.
There are multiple ways to be “unconscious.” Head trauma, sleep, general anesthesia, fainting, coma - for example.
The experience varies wildly: from absolute nothing under general anesthesia to extremely vivid stuff during sleep.