It's extremely hard to join large structures airtight, with joints being stronger than the material itself.
Soviet, and Russian spacecraft, and modules were traditionally made with extremely uneconomical method of machining the vessel from a single giant piece of aluminium to not to worry about joints, and their strength under space conditions.
The difficulty is not about joining large structures airtight. otherwise we would have trouble with building commercial aircraft.
the problem is when you want to make something out from the absolute minimim weight possible due to economy of putting things into space, then you can't do with any connections whatsoever.
The pressure of -1 bar isn't particularly challenging engineering wise.
Soviet, and Russian spacecraft, and modules were traditionally made with extremely uneconomical method of machining the vessel from a single giant piece of aluminium to not to worry about joints, and their strength under space conditions.