It's not a big mystery. Green tea has the most amount flavanols. Fresh leaves also have more compared to aged leaves. Also, powered forms of tea release them more quickly compared to loose leaf. However, you could just steep regular leaf tea for longer as well. The key thing to remember is to not use boiling water, as that may degrade the flavanols, so select a temperature just-below boiling (80-90C / 175-195F).

I'm not in the right headspace to read any studies on it but based on my understanding of how food works on a chemical level I'd say it's a safe bet to say that the difference in preparation is mostly down to heat and steeping time.

If you steep for longer it gives the water more time to absorb flavanols.

If you use a temperate that is too high you'll denature or destroy the flavanols, whereas if you use a temperature that's too low they may not dissolve into the water as readily necessitating a longer steeping time.