Okay so this seems helpful (thanks a lot!) but it is a very lengthy tutorial so I figured for everyone who's curious here is the short form:
CurX here is the right boarder (BBox.right) of the last placed scene. CurY here is the Y position of the last placed scene, in this example it always stays the same but if you want to add verticality to the game you might want to change this.
For adding a room (in this case called "scene") above the current room, you need to take BBox.top for the Y position (curY) and if wou want to add a room below the current one you need to use BBox.bottom + NewScene.height instead.(this wasn't mentioned in the video but I believe this should work)
Now, you got a family with all the scene objects (so basically every object in your game's world), in this case it is called "Entities".
Now with this funky formula you set the position for each entity to the position where the new scene is going to be but add the own position of the object relative to the scene position (so self.x & self.y) as an offset.
For this formula it is essential (wasn't mentioned in the video though) that the origins of the background scene objects are in the lower left-hand corner.