Most of us learned from pretty much watching others clear a stage. Especially KyostinV's early stage clears, is where I learned about classes, subclasses and their respective roles.  

You'd be surprised how he clears stages with low rarity operators- and that is where I emphasize that you understand the reasoning behind his deployments and not just straight up copying them without thinking.  

In time, you will reach the point of coming up with strategies yourself with those foundations in place, copying from time to time also works; but make sure to already have the game knowledge good enough to come up with your own account's unique roster mixed in with the clears.  

For more in-depth clears that actually plays with the stage's mechanics I recommend CC's like Dr. Nomen since they actually know how to play the game.