The house is built air-tight, does that mean I can’t open the windows?

Windows can be opened in the house just as you would in a normal house, its just that there would be no need to in terms of fresh air provision since the MVHR constantly changes their air in the house to ensure that it is always kept fresh. If you opened a window this process would still carry on, it is just that if you opened the windows in the winter, you would lose heat to outside.