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Your question is not very clearly.Maybe your meaning is that the buck (or body) of nMOS connect to Vdd and the buck of pMOS connect to GND. what will happen?
The buck of nMOS is P type, the source and the drain of the nMOS is N type, generally we connect the buck to GND, and the source and the drain connect to a higher voltage than GND. Then the PN junction will be reverse biased, and no current flow.
If we connect the buck of nMOS to VDD, the PN junction between source-to-buck and drain-to-buck will forward biased, and there will be a very large current flow between source to buck and drain to buck, which is not allowed.
The reason for pMOS is the same.
We connect substrate terminal NMOS to lowest potential and PMOS to highest potential because we need to reverse bias the drian source terminal with respect to channel and substrate and moreover if we do the reverse it will work as week buffer as when Vin =0 nmos will work and gives 1 at output and when Vin=1 then pmos works and vout =0 so it works as a buffer.
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