flying wing with tricycle gear

So I am kind of stuck here. For hours I tried to get rendering and import correct. How do you even debug stuff effectivly in Blender. Please download and tell me what I did wrong. Here is the version with all modifiers applied. Looks good in sketchfab, but is hard to edit: https://sketchfab.com/3d-models/flying-wing-with-tricyclegear-modifiers-applied-f97aa67b86cb47a0afbf74e1c22160c7