Is it true that when u put distortion on fender strat, the amp will hum?
in general, NO
where ever you heard that from, its either misinformed or generally, clueless.
Distortion dont add hum in general. If theres any hum thats already present, distortion will just "magnified" this hum and amplified through the amp loud enough for us to hear it.
In the case of a triple single coil strat, the hum is inherent of the single coil pickup. Thus when distortion is insert between the guitar and amp, it will bring this inherent hum(from the single coil pickup) up to several time and amplfiy through the amp.
In general, distortion dont add hum to anything, it is just multiplying the clean signal many times till it get distorted. If theres any additional noise/hum when we used distortion, its because the characteristic of distortion is multiplying the sound (be it pleasant guitar sound or other noise/hum thats all in there, but not obvious when in clean signal stage) many many times and thus bringing it to audible level.