Hellow this is my First Guide ever :3 I go by Strix or Dragon.
English isn't my First Language, Please ignore Typos :x
This is a small Guide i crafted for Zeri.
I will try to explain everything in here.
Zeri can be in the right hands a Strong ADC even if she is a Scaling Late Game Mosquito.
If you have Any Questions about this Build feel Free to ask me :3

With this Build Zeri get's an Early Power Spike with two Items.
But it is a bit Squishy compared to Bruiser Zeri. Kiting and the Knowledge when to go in is Important!

Try to Focous on your Farm. Since you are Late you want to get your Shiv and Inf Edge early (With Boots). If you see an chance to Poke go for it but try to avoid Early Fights. You can go for Kills if the Enemies makes a Mistake or your Support opens a Window for you but "Better save then Sorry" :D

Statikk Shiv > Infity Edge > Hurricane Is the Core Item. it gives you enough Attackspeed + Damage + Crit
Mortal Reminder / Kraken Slayer / Lord Dominik's Regards if you need the Anti Heal or Burst Damage or to Shred Tanks
Last Item Depends on what they have as a Team Comp or Champs in general. (also whos the most threat for you)
Mercurial Scimitar / Guardian Angel / Kaenic Rookern

Don't be scared to Back if you miss a little bit of Gold ! This is why you have Future's Market for ! Yes the Rune puts you in a Debt and can be Painfull but getting a Item Lead is way stronger and can be abused against the Enemy.
Going for Noonquiver and Berserker's Greaves early gives you decent Damage for Poking/Short Trades and Easy Farming.

Well yea.. But the Defensive Item Helps you alot in Team Fights. Since it's an AP One-Shot Season i strongly recommend Kaenic Rookern. It makes it Impossible to One-Shot you ! In some Games i go Kaenic Rookern third after i finished Statikk Shiv and Infinty Edge

You don't want to be Tanky with this Build !
There's no need for. You can kite every Champ in Late and Deal Enough Damage to Carry.
Why Future Market ? Helps to get Items Faster.
Why Cosmic Insight ? Well it Does NOT Help for your Core Items that might be True but for the Last Item that you build it's very Helpfull + the Low Cooldown on your Heal and Flash will help you in Teamfight.
Its just simply better to have Heal earlier up than the Enemy !
Also Flashing at the same Time as the Enemy is super Worth for you, you get your Flash earlier than they do >:3