Depending on the world layout, the level JSON will change:
Linear horizontal or Linear vertical: the levels array order reflects the world layout.
Free: each level has worldX,worldY coordinates that locates it in the world.
GridVania: the worldGridWidth and worldGridHeight values in JSON root define the world grid dimensions. Each level has worldX,worldY coordinates that locates it in the world, and these values are snapped to the world grid.
Neighbours
To make your life easier, in each level an array called __neighbours lists all nearby levels (ie. levels that touch the current one). The content of this array complies to the world layout setting.