do some factory heater cores flow hot water thrue them even on 90 degree days
Some do. In the old days, there was a valve in the hose that was operated by pulling a knob on the dash. In many of today's vehicles, water flows freely and air either goes through the core or it bypasses.
they all do now, the air is changed by the heater actuator and heater blend door, with a/c on the door moves and blocks off hot air, when switch to cold the door moves and shuts off the hot air, that way vehicles with auto temp will keep inside temp at set degree at a/c heater controller, are you having a heating or cooling issue with your vehicle ?