This is a feature of the game known as embarking. It wasn't in Civ games before Civ V so you might not be familiar with it. In Civ VI once you research, I think its Ship Building, all of your units are able to enter water tiles like is demonstrated in your video. So unlike earlier Civ games where you would need to build a transport ship to move land units over the ocean, in Civ VI the land units can move themselves across water.