I looked up the lens in the Vade Mecum. The Doppel (the "Objektiv" part just means "lens") seems to be a doublet, as the name would suggest; the text says it dates "at least from 1902", is found in an f/11 version (which seems to be what you have) on something called a Simplex from 1926, and "was often the low cost option on cameras such as the Heag 0, 00, 1, 11 of the period".

It looks like the Simplex was a low-end 9x12 folder, which is what you have---I'm not sure whether it was an Ernemann model or existed only after Ernemann was assimilated by Zeiss.

These old folders are pretty fun cameras, but don't expect any miracles out of the lens. I think Freestyle is the only US source of 9x12 film. Plates turn up on eBay once in a blue moon, but to shoot film in it you'll need a sheath that fits inside the plate holder you have. In principle they'd be easy to fabricate out of sheet metal, but while it's obvious how to do that when you see one, I don't think it would be so easy without a model.

The different back formats are supremely complicated and are probably the main topic of discussion over on the plate camera forum. It looks to me like your camera can probably share holders and backs with Voigtlaenders. In my experience, the easiest way to accumulate film holders and backs is to lurk on eBay for dodgy cameras with good backs. And then there you are with a bunch of plate cameras...for good or ill.