New to the hobby, rules question

Hello, I watched a Dungeon Dudes video about Monks and they explained that a Monk could take a feat in Great Weapon Master or one level in Fighter to get the Great Weapon Master fighting style and that will allow your Monk to Dual Wield. What I could understand was that this would essentially give you an extra attack with your off hand (or maybe just make it more effective?) would someone be able to clarify if that is how this works (or is worth it). Also depending on the answer, does anyone happens to know if it work the same way in bg3?