Harmful effects of microwave go back to original designs many years ago that leaked microwaves outside of the oven. That has been corrected for a long time...Newer microwaves do not leak. As for effect of microwaves on food, they cause the water in the food to heat up and cook by steam. There is no residual effect once shut off.
See this statement from the World Health Association https://www.who.int/peh-emf/publications/facts/info_microwaves/en/