Going only off of the video I saw of Tim Cook with the shield...
The entire shield is a clear plastic film. It’s held against the forehead using a fabric strap. The shield appears to be either die cut or laser cut, which explains the production speed. A lot of the face shields shown online are 3D printed with hard plastic components, which is a very slow process.
The material looks a lot like the clear plastic film that wraps an iPhone or even an iPad charger. The shield folds together in the same origami-like way. Based on comments about the supply chain being involved, it seems like they leveraged the existing manufacturing process for Apple product packaging and thickened the plastic.