So i just got Medieval Twitch
So I recently got Medieval Twitch and I don't know how to build twitch. I'm a support player so I'm not familiar with ADC builds and the situational items like when to buy 
and etc. So I looked at some guides watched a few pro plays and something I wanted to ask was
- When or Should I even go
in some guides they don't have and on op gg the highest pick rate build doesn't even have a 
seems like a core but sometimes people build this early while others build it after 
basically: when and why to buy
is this a viable item is so when and why to buy- +
so for this one Doublelift in one of his soloque games went + and didn't buy runnans. I think he went this because the enemy had little tanks and more squishy so the sttatick proc would be more vaubale and got so he can have more procs. Looking at this it gives 60% crit and with that's 80% and with boots it's a fair amount of attack speed. Is this a viable option or was it just him trying something out?
Overall, I just wanna know the situations in which you change your core and adjust your item build and weather things like this apply to 
Thanks in advance!