The Rav Moshe below rules that one could not wrap something in foil and place it on top of pot (or hot plate or blech for that matter) because of hatmanah. Is this how we pasken? Thank you.
That is not how we rule. First, if the aluminum foil is only to prevent the pot from getting dirty, and not to keep the heat, then it is not a problem. See Shulchan Arukh OC 257:2. Also, if you are doing it to preserve the heat, the Chazon Ish (Shabbat, end of no. 37) states that hatmanah only applies to something that the vessel is wrapped in, not the thing itself that you wrap the food in, that is like the vessel itself.