a know it all
A know-it-all is someone who acts like an expert even when they clearly don’t know what they’re talking about. They make it look like they’re super smart, but everyone can see through their fake knowledge.
'I’ve studied this subject for 10 minutes, and I know more than you!'
'I’ve read one book, and now I’m an expert on everything!'
'I watched a one-minute video, and now I know more than your teacher!'