Unity and Blender Normals


Hello people,

I am confused. I've used the "FBX exporter" from the package manager of unity to export this tree on the left side to Blender to create LODs. When I export back to Unity, the same geometry with the same shader and texture looks like the tree on the right. The internet tells me, that I have to write my own shader, so that faces are getting renderd from both sides. So why is the left tree the way he is?

Asset Source:
https://assetstore.unity.com/packages/3d/vegetation/trees/mobile-tree-package-18866#reviews