these doors built how?

[Promiscuos Monkey]

there are 9 doors for sale.
how do you think they are constructed?
cardboard in between? styrofoam?
they cost 1000 for 9 pcs
either they are insulated or I have to find a way to do it.
It would have been a cheap way to get camping walls for my car project
fold-out sleeping alcoves on both sides

 

[Elsparv]

Similar doors at home and look like toilet paper rolls on the inside with a frame of wood (low quality):
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