Do most people expect me to know the fights beforehand?

No. Fights along the main story as well as the Alliance Raids are all designed as casual content which can be cleared with entire groups of average players. They leave a lot of room for recovery as long as you pay attention and learn the encounters as you play.

The further along you get, you'll start to notice that their design philosophy for basically every boss changed to show each of their mechanics on their own to allow learning and recovery before rotating through or mixing mechanics together.

is there a point in the game when I should go back to watching a video before I queue?

Anything that is not Extreme (also includes content with "The Minstrel's Ballad" in the title), Savage (this includes The Binding Coils of Bahamut for ARR), or Ultimate content does not necessitate doing research beforehand.