These are parallax textures, or underground, just create a texture according to your need and just fill it with hidden collisions and parallax effects (layered textures).
Your character will move over the texture, respecting only collisions.
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.
1.You can use Ysort ~
2.Maybe layer and mask setting can help you，You can set different layers on the elements and use mask to highlight the hierarchical relationship。
