90 extraction of loose solo upper back molar with decay under crown today. How to keep area clean? Brush, peroxyl, what? Dmd asst didn't say what to do

Brush. Brush and floss is the best thing to do. I'm not sure what 90 is? Ultimately, it sounds like the crown needs to be replaced or tooth extrated. You may need a root canal as well... If unclear ... Call your dentist.. I don't mind patients calling me for clarification of my instructions.
Postop care. You should have been given postoperative oral hygiene, dietary and wound care instructions. These include salt water mouth rinses every few hours and no brushing for a day or two. You should be on a soft, non-chewing diet. Peroxyl is ok but probably unnecessary. Don't brush the actual tooth extraction socket; it will heal uneventfully by itself. Good luck!
Do not disturb . Keep pressure on the area with gauze packs, and try not to spit, rinse your mouth and of coarse no smoking , keep the area clean and do not disturb the blood clothe that formed in the socket where the molar was., because that heals into bone later, on the second day you can rinse with warm salt water to accelerate healing.