The Hatches needed to be Manually Opened and Close.
When The Hatches are open you have the Option to control how exposed your crew can be, like Exposing only the head, head to arm Exposure, and finally upper torso exposure.
Depending of the tank, the each crew should be able to open and close their hatches and if possibe shoot from there.
If by chance the crew got killed while using the hatch is open or you forget to close the hatch, the enemies can kill the crew by shooting through the hatch or throwing a explosives or molotov in the tank.