Hello,
It is a standard considered in electronics that 5V power supply is logic 1 and below 1V is logic 0. This because the ICs and other equipments can handle minimum of 5 V to work . If it goes higher than the LED will burn out same goes to other equipments too. If they get any voltage below 1V it won't work and it will become dead which is a sign for 0 logic.
It is NOT gate. If any value less than say 1 becomes 0 and any value is greater than 5  becomes 1. There is a threshold value in it which is a constant. NOT gate only knows that logic value and reverses it in output.
Hope it helps you

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