I like this definition the best. If someone is making a super complex sandwich with many ingredients and passion, then I’m fine to call that cooking. Same with a cold soup, a cous-cous salad or a fancy appetizer. Many dishes in top notch cuisine are served cold. In molecular kitchen, there’s even stuff served below freezing. Still all cooking to me.

If someone just warms up a can of Ravioli, microwaves convinience food, etc. I’d consider that rather food prep. If using the microwave is just one step of multiple in a recipe, than that’s fine again.

For me cooking requires a minimum level of effort rather than a minimum level of heat.