Alright this is a great question,

I don't buy Greaves all the time, I still think it's a good item though, I abuse it when I'm ahead, if you're ahead one kill go Greaves first item, not zeal, not other items, go Greaves, it offers too much for Yasuo and can be used as a cheese in lane, you will win every single trade if you stack fervor fast, do not go Greaves first if you are going Bloodlust.

I think randuins is only good vs Tryndamere, Jhin, Vayne, Caitlyn or Twitch, these are the main critting ADCs, sometimes if they cannot play ADC right, I do not go randuins because I would prefer bursting them if someone cannot peel for them, so I don't think randuins is a core item.

2 defensive items with those items above are actually amazing, those 3 items already grant you incredible damage + fervor, getting defensive items are great, but I would prefer going defensive items like Maw/Steraks, Randuins, Deadmans, Mallet, etc.

I build Triforce whenever I like tbh xD
It's viable and such, it gives great stats and used to be one of the strongest builds I used to abuse, right now some Yasuos still use this item, I would go triforce first if I was laning against someone easy at top lane that cannot trade well enough with you, I would go PD if you cannot win as much trades, Lament offers just too much.

It's not that I build armor, that's a standard example build, I always go merc treads when they don't have AD or they're more so AP, facing a mage I will always go treads since it offers a lot, I go Tabis when I am vsing a Talon or Zed Preferreably.

Hope this Helps!

Thanks Liilly

Thanks freshsteak69 whos a bronze scrub hehe xd

If you're talking about the core items, 100% of the time I recommend going the zeal item first, then 90% of the time going infinity edge, the reason why I don't always go infinity edge 2nd is because if you're not at least snowballing or doing good at all, you might want to go for some lifesteal and some decent damage, bloodthirster for example.

After bloodthirster/ie go whichever of those two you haven't purchased yet, after its very situational, if you grabbed a hexdrinker early on, try to finish it after your Bloodthirster/IE, after finishing your hexdrinker into a Maw it really depends from here, if you want more lifesteal but you're getting CC'ed to death in a teamfight you might want to grab a Mercurial or the QSS Upgrade, some other options other than bloodthirster would be Mercurial, BOTRK or Hydra, I recommend going the first two, as they offer more unlike ravenous, However ravenous is extremely situational, I almost never go it since I rather go items that give me more, If you're ever vsing someone like a Sion,Maokai or someone you think is super tanky and unkillable, I recommend going BOTRK and Lord dominiks, trust me once you ult off them they will be as squishy as ADCs, I hope this will help you and good luck.