did you find the first key in the first level?
The first key opens a door > which contains another key > which opens an door somewhere in the house to a room with another key > which helps repeat the cycle of opening doors. If you "can't open" any of the doors, there's a chance you're missing a key.
This is what I call the Locked Door Fallacy.
The first door always has a key, that unlocks another door that grants you access to the next key, and then the next, and then the next, and then the next until you exhaust all uses for keys. If you run out of keys to use it either means:
1) You haven't fully checked the area you previously unlocked. Or
2) You have the last key for the exit out of level 1, the kissplant household.