How would i make a smooth level transition?

Hello everyone I was wondering how would you make a smooth loading transition like this RE77 | Gameplay - YouTube he goes through the door and it loads the next level but smooth so it doesnt freeze and have to wait for it to load because it ruins the immersion. I know it has something to do with a trigger box and next level but other than that im stuck on how to make it smooth.

This tutorial should help you out. Epic has lots of useful tutorials on their youtube and twitch.