EGCG from Green Tea is more potent than Quercetin - with brewing guide

Taking just ECGC can harm your kidneys or liver because there are compounds in the tea that balance that out that aren't there when you just extract the EGCG. Same with the bark they originally used to make quinine, and probably with grapefruit peels too.

Not all Green Teas have EGCG. There are exceptions but generally you want a Japanese grown leaf, harvested later. Sencha probably the best and not too expensive. Gyokuro is harvested earlier IIRC and it's expensive.

I bought some at Costco that are a blend of Sencha and Matcha, the instructions are put the tea bag in the cup, pour 80°C water on top, steep 30 seconds and shake it out. I steep it covered and squeeze it out too (careful it's hot). Three ways to get 80 degrees, from most to least confidence: 1. get an electric kettle that lets you set the degrees, 2. boil not quite all the way and cool for 3 minutes. 3. boil until you get medium sized bubbles: not large and not really fast.

Just boil it once, every boil you release more oxygen from the water.