mesh going transparent

im having a few problems when uploading a model from Blender to Unity. some parts like inside the hoodie or Arms are getting Transparent/Invisible or see-through. anyone knows what i can do to either prevent it or fix it?

in Blender: Screenshot - 886044aa47f9a1942e1f8bef62c1599e - Gyazo

in Unity: Screenshot - 5117c841556627c19bdfd5e560c66d15 - Gyazo

Thank you in advance!

PS: I am not the Owner of the original model

In blender it show in standard settings both sides of a face and not only the front face that unity is showing.
Enable backface culling in blender to show how it will look in unity.
https://docs.blender.org/manual/en/dev/editors/3dview/properties/shading.html?highlight=backface%20culling
Unity will only render the front face of the polygon (the direction the normal of the polygon is facing).
Option 1: Fix the model by adding faces in the same position as the outside faces and reverse the normals to face inside.
Option 2: Disable the culling feature inside the used shade. This can work but mostly is hacky and will cause lighting issues on the backside faces.