Why does unity keep messing up my blender models?!

I have forever been struggling with unity messing up my models that I made in blender. But this time it really got me. I was working on a simple sniper rifle scope and when I imported the model into unity, this is what happens:

However, in Blender, it shows up properly:

I don’t know if it’s because I deleted the faces covering the scope instead of extruding it inwards? But right now I’m just pulling my hair. Any ideas or tips?

Looks like your normals are flipped. Blender displays faces two-sided by default, Unity, as is the rule for game engines, does not. So Unity doesn’t mess up anything, your model is messed up - but in Blender, you don’t see it.

However, you can see your normals in Blender. Follow this article.
You can flip them in Blender as well. If my initial assumption (flipped normals) wasn’t wrong to begin with, this should fix the model for Unity.