whats this part of a tap called?

i have a vanity with a sink on top, the taps and spout are mounted in the ceramic of the sink itself. i have 2 taps on the wall where the water to the sink can be turned off.
i want to know what the part under the sink is called that the water supply goes to from the bottom where the output of the bottom part of this tap is at right angles which goes to the spout. anyone know what the bottom of the tap is called, the part that houses the tap seat?
can you get these selerately? if so anyone know about how much they cost??


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