Rather than using a grid to lay out the exact position of every unit on the battlefield when that's constantly changing, we use zones. A zone is an area of terrain. It can be a room, an area of forest, a building, a ship, a meadow, a patch of sky, the back of a giant monster, or any place really. Zones usually have borders but in a pinch distance can define a zone. 

Melee attacks happen within a zone while ranged attacks happen from one zone to another. Destroy attacks affect the zone itself, destroying buildings and smashing terrain. To damage terrain you need to roll higher than its rating. If you do so, its rating is reduced by 1. Once its rating is reduced to 0, the zone is completely destroyed, which also destroys all amenities and ruins anything you wish to ruin.

Units that spend their entire turn in a structure not affecting anything outside of it may choose to be in full cover, which means they can't be hit by ranged attacks from outside the structure until said structure is destroyed. 

Mobile structures choose whether they are influenced by social skills or not. If not, anyone running the structure with an operate skill can control it. There are advantages and disadvantages to this.