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

If someone just warms up a can of Ravioli, microwaves a convinience dish, 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.