Insulation resists heat transfer in both directions. Extremely well insulated and air sealed homes resist heating up just as much as they resist cooling off.

Windows are a bit special in that you can get solar gain through them, but modern windows can have coatings that filter UV and IR from entering a building.

People and electronics are heat sources inside a home, but pretty minimal ones. Any reasonably speced ventilation system a modern home needs to exchange outside air will easily handle keeping a modern house cool.