Half the breakers are connected to one hot, and half are connected to the other hot. For 120V you wire hot/neutral to the receptacle, while for 240V you wire hot/hot. (Plus ground, of course.)
In the EU, receptacles are wired hot/hot, and there is no neutral conductor.
The typical EU configuration:
The transformer delivers a neutral and three phases, in a star configuration.
That means you've got L1, L2, L3, N and GND.
N to any L is 230V, any L to any other L is 380V.
That also means a typical grounded socket has e.g., L1, N and GND, so a neutral and a hot.
A high-power socket or e.g. a stove will have GND, N, L1, L2, L3.
My stove has the oven running on L1 and N in a 230V hot/neutral and the stove at L2 and L3 in a 400V hot/hot configuration.
(Belgium is the exception, having phases at 113V off the center point, so sockets are in hot/hot to get 230V between the phases)
Here in the US, where split-phase is the residential standard, a house with three phase is quite rare. The HV lines running on poles in a neighborhood are mostly single phase, at least in rural areas like mine.
