What is the hex code transparent?

Hexadecimal code represents values of colours. Digital media rely on additive colour mixing, which is based on light, not pigments.

Even if you input the code to add '00' values for all of the colours, you will get black, because you are telling it "do not add any colours of light", and the absence of light is darkness (black)

There is no code in hex that means 'No colour', because what you are actually telling it is 'No light'.