I don't play Xin Zhao often, I have bad macro, bad mechanics but every time i use this bot lane turns into a 2 v 1. I find this build is very easily countered by CC which stops Xin's sustain from his lifesteal. It is NOT a build to play into ranked although it is a fun troll pick. His early game is very powerful if you can isolate their ADC. If you happen to make it to 35+ minute mark it is too late (s2021 might be different idk) and you have lost the game as Xin Zhao offers very little in teamfights unless his ultimate is used well. He can capitalise on his power spikes and early game because of not just one but two free kills. Xin Zhao does very well at clearing (stealing) jungle camps late game due to his passive and item build.

Notes from SMURFING10142069
-stormrazor gives u a slow along with w so they cant run easily
-with crit u dont really fall off later into the game
-pick people off one by one and if u get a good teamfight
-go solo drake
-u can solo drake with just the stormrazor lol
-thresh and blitz r p good with xin
-if they bring the enemy adc or support to u u can just get a free knockup after the supports without having to dash so if they flash u can just dash to them or slow then dash

Special thanks to SMURFING10142069 for providing a build and rune page that actually plays out like an ADC

I CAN'T GUARANTEE THIS BUILD WILL WORK IN s2021 BECAUSE I HAVE NOT DONE EXTENSIVE TESTING
pls feel free to leave suggestions