If anyone wants to know. Here's what I found.
It is 2D modular asset. You draw sprites for ground, corners, platforms, etc. and then place them on the scene (all these sprites should be in the atlas for better performance). That is, it is something like a tileset, but without a grid.
Also you make atlas for details. Background and foreground can be a single image or make with sprites.
For collision you may use tileset with one tile (i use black box 32x32) after that you just put your sprites on top of it and hide tileset. Also this tileset help you with mesurment.
May be it's not how those levels was make. But i'll be doing in this way.