If I have vents/ whirly birds in the ceiling or roof for ventilation, do the insulation installers cover those or are they left open?
They should be left open.
At first it may seem odd to add insulation for warmth and then purposely allow cold air to enter the attic through vents, but this combination is the key to a durable and energy-efficient home.
In the winter, allowing a natural flow of outdoor air to ventilate the attic helps keep it cold, which reduces the potential for ice damming. Proper insulation and air sealing also keeps attics cold in winter by blocking the entry of heat and moist air from below. In the summer, natural air flow in a well-vented attic moves super-heated air out of the attic, protecting roofing and removing moisture. The insulation will resist heat transfer into the house.
The most common mistake homeowners make when installing insulation is to block the flow of air at the eaves. Never cover attic soffit vents with insulation — the installer should use rafter vents and soffit vents to maintain airflow.