It depends entirely on what is being cooked, and the dish it is going in. Onions, FI, sometimes are cooked at a low heat, just enough to soften them, while other times they are cooked at higher temps to caramelize them. Usually, garlic is added only on medium heat, or lower, for 30-60 sec, so that it doesn't burn, but there are some dishes in which the garlic is browned, and the bitterness goes well in the dish. There are a number of temperatures given to heat meats to, but different temps to use to get them there. The only general temp I could give is for boiling water, for something like pasta, using a high heat!