its an indie adult fantasy game, the game itself is a mix of visual novel, people management, rpgs and adventure games, and as with many games you get levels by gaining exp by defeating enemies. Examples of these games that handle leveling this way are Rpgs like fire emblem and dragon quest to name a few.
Viewing post in First quest impossible in all male mode?
I have not played any Fire Emblem or Dragon Quest games, but it doesn't appear that those games even have quests, so... of course the quests don't give experience because they don't exist. I guess maybe Sandbox Mode (which made no sense at all to me initially, because I figured the game was already sandbox, being purportedly a management game) is more like those non-quest-based games. If that's the case though, it's really poorly tagged, and the Story Mode is incredibly poorly conceived. Should just be under RPG, not Management, and shouldn't even have a Story Mode.