I was talking to a guy about graphics cards, and he told me that the undervolt can damage your graphics card is this true? i have 5700xt
Undervolting doesn't damage the card since you're talking millivolts difference, quite the reverse since it reduces heat and stress, and will increase performance due to it running cooler enabling higher clocks for longer periods. It will, however, cause it to become unstable if you drop it too far, so do it a little at a time and use 3DMark to watch for artifacts.
he told me he could make a short circuit .
in my 5700xt undervolt 2000 in 1.1 play graet but rarely cause black screen on driver issue
That doesn't make any sense whatsoever...
Too low voltage can cause crashes. You need to drop frequency a little or increase a voltage a little and retest again.
he claims that when he gets less power for a long time, that it can because it gets less power to short circuit the chip
A short circuit is when power takes a shorter path than intended, a condition which cannot be created by using lower voltage levels.