They're also called, or very similar to, "carboys". Google says they're called "jimmy johns" in USA, but I've no knowledge of that.

demijohn are a bottle in a wicker wrapping. Once you get to large glass bottles like this breakage is real issue so it makes sense to wrap them so they can be moved with less chance of breakage. Note that if you do build one of these, be VERY careful moving them as they can break with a small slip and the large pieces of glass can easily cut tendons, leading to lots of long term issues.

Demijohn is just a large glass bottle in en-gb usage, usually with straight sides and circular handles, holding just under 5 litres. Example: https://www.wilko.com/en-uk/wilko-demijohn-container-glass-4....

They're also called carboys in the US. Nice brewing shops will have a variety of sizes and shapes to choose from.

Thanks! I was curious about that myself. They seem quite pretty and a nice size for an 'experiment' like this.