Picture everything that has to exist before water comes out of your tap: a store of water somewhere high or somewhere large, pumps that push it along, and running under every street. None of that is the water. It is the machinery built around the water to make it go where people want it to go. That built system — plus the power to move water through them — is the picture the word holds. Once you can see it, the word can be pointed at one building instead of a whole city, at a garden instead of a town, or at the inside a person.
From the word’s history
Wiktionary gives the word as water plus works, where works means machinery or a factory — the same works that makes steelworks and gasworks. So the name says plainly what the picture is: the works built for water.
字典記載這個字是 water 加 works，而 works 本身就有「機械、工廠」的意思，跟 steelworks（鋼鐵廠）、gasworks（瓦斯廠）是同一個造字模式。所以這個名字本來就把畫面說得很白：為了水而蓋的那套設備。
Wiktionary, waterworks etymology 1 (source_ref kaikki:3f6f69a190036ae4:909602697): "From water + works (machine, mechanism; factory or factories)."
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