Sorry, I know it's not very clear right now, I will improve the tutorial at some point to make it much more transparent, just feels like a waste investing too much time into it right now if I end up changing some mechanics and make the tutorial obsolete.
So basically you have a certain number of rooms of the dungeon unlocked, rooms of one floor are underground sprites that extend horizontally to the right, after you have 10 rooms, next floor is unlocked and the process is repeated again.
When heroes choose what quest to do, the first check which quest is being done by less heroes that maximum you've set. Then they check if there is "available" room on the floor, and if yes, they lock this room for their quest and make it "unavailable" for everyone else until either quest is completed or they die.
So essentially if you have 10 rooms unlocked on floor1 and 5 rooms of floor2, you can have 10 heroes at maximum doing floor1 quests (gathering mana crystal, leather, wood, stone) and 5 heroes doing floor2 quests (iron ore, clay) at the same time.
The hotkeys for buildings I also considered, that could be a good idea. And indeed, potentially adding text label to number the rooms might be good.