I the case of a flatbed scanner, photocopier, or MFD (Multi Function Device) the glass where you place the document is the platen. With the DIY book scanners they are using two pieces of glass/plastic in a V shape (or one piece bent into a V shape) to accommodate an open book in a V shaped cradle. The V shape platen will flatten the pages to allow a clean non distorted way to capture an image of the pages.
The platen does not filter out the reflections.
Yes you place the V platen on top of the book.
The platen is moveable normally up and down to allow the pages to be turned.
The size of the platen will only accommodate books smaller than the size of the platen.