I can't find the file which is using up storage on my Mac OS, s transferring WhatsApp via dr fone and my disk says full however there is no backup file I have individually gone through all files and cannot find a 40 GB file anywhere

It is possible the app transferred it as multiple files which total 40GB (maybe less if the transfer failed). It is also possible the data was deleted once the app realized there wasn't enough room for the full transfer. Are you sure the data should be located in the "Documents" folder?

You can use OmniDiskSweeper to easily locate the location of the largest files & folders.