so the arduino lets me send 5v to my sensors.
Voltage isn't sent. It is a potential that is provided. Current isn't sent either, it flows.
does it matter if the upper threshold (3.2v, 5v) is different for different parts of the circuit
If it mattered, almost no electronics would work....
Ground isn't really a "comparison", it is the "reference." All components need to have a "common" "reference" (quotes added because those are other words for "ground"). Ultimately you are trying to make current flow between a higher potential and a lower potential. To make life easier, we commonly call that lower potential "Ground."
So if your regulators and all components they are connected to share the same ground (reference), current will flow.