Its other name is polyurethane foam, normally they use it for insulation, so it's a bit crazy to use it for making heads The base was a balloon covered in paper mache. You can put the expanding foam onto paper mache, then wait for it to dry, so you can start carving the head's form.
There are many tutorials about expading foam, it depends on what you want to use it for, usually cosplayers use it for weapons and props, because its light weight. You can use it for heads too but only if it has openings. Sorry I'm not good at explaining, I hope it helps ._. Good luck with experimenting