The question “Since advantage and disadvantage caused in a heavy obscured area cancel out, what effect does it have in combat?” notes that attacks in darkness or magical fog cause attacks to be cancel out advantage and disadvantage - thus “leveling” the battlefield as no one has advantage or disadvantage.
Additionally, heavily obscured areas are described functionally equivalent to invoking the Blindness condition.
Adding to that, the PHB p.291 says:
“An invisible creature is impossible to see without the aid of magic or a special sense. For the purpose of hiding, the creature is heavily obscured. The creature's location can be detected by any noise it makes or any tracks it leaves.”
What seems unclear is whether the location of an opponent is automatically known or not known in heavily obscured areas - and subsequently how attacks are made.
When in heavily obscured areas...
Does the PHB reference above imply that, when using battle maps, the combatants in heavily obscured areas automatically know in what 5 foot square combatants are located (i.e. hearing their location)?
If combatants do know each other’s location, what happens if opponents simply stand still? Are they automatically unfindable? Or do combatants always have to roll a Stealth check to Hide in what 5 foot square they may be standing?
If combatants do not know where opponents are located in heavily obscured areas, then how do combatants ever make an attack with the modifiers mentioned for the “Blindness” condition? Do they randomly choose 5 foot squares to attack and automatically miss if they choose the wrong square?
If combatants do not know where oppponents are located, how do they ever find each other? Do they have to roll a Perception check every round to know what 5 foot grid in the battle map to attack? Is that Perception an action, bonus action or free action?