every state is usually a little different because of different weather conditions. like on the west coast you have to use flex water and gas lines at water heaters where i believe in places like Illinois (not 100 percent on that) they're illegal, but there is an international code which is the basis for all state and local codes. Everything has to meet the IPC but state and local ad additional restrictions as long as they dont circumvent the IPC.
Also, what your probably seeing is pex. they dont really use cpvc for freshwater lines anymore, only sewer. copper is usually found in older houses or new construction where people are willing to pay extra for it, but it's hella expensive and people will go into your crawl space and steal it.
also, its not uncommon to see copper in places in a house where the pipe has to be rigid, such as showers and stub outs where your faucet connects to under your sink and then have pex behind the wall on the long runs.
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