[I'm New] A Game Like Chain Chronicles or Grand Sphere

Hi. I want to develop a game that’s like Chain Chronicles or Grand Sphere. The game has a world map scene to choose your quest from, a town scene to summon heroes or to sell/buy items, and the fighting scene gameplay. The game is a sidescroller-ish mobile game.

Now, I’m still learning UE4 so I don’t know the exact approach for this and I frankly need guidance. So here’s my noob question: do I put the town, world map, and fighting game play in one level - creating a stack like structure of different scenes and setting their hidden state when they are needed (ie. fighting gameplay on the far back, summon in middle, town in front)? Or do I separate the scenes into levels and call them when they are needed?

Also I have another question regarding the Strategy Game demo. Were the main menu and loading screen different levels?