They do both, decrease brain activity and make the remaining activity more interconnected. Psychedelics has not been found to increase brain activity anywhere, if they do anything to a brain area, they only decrease activity. Meditation also decrease activity
Which psychedelics? To my knowledge, no study has ever shown increase in brain activity on psychedelics, only decrease. This is the paradox - your brain is going to sleep but you have the most crazy, rich, meaningful experience of your life. Biggest argument against physicalism imo