Tooshkit has posted a great little guide on finding the Nemesis Exotic Sniper Rifle. There's lots of Exotic weapons in The Division 2, and this is the last to be located and created (it actually has some time-gating). Here's the details.

Nemesis (Sniper - Crafted)

• Blueprint Hidden Quest
• Attributes
• Preparation Talent: Only works with weapons which have a scope equipped.

How To Get All 4 Parts

Part 1. Inside the Black Tusk stronghold Tidal Basin, you need to pick up a golden key on the first floor of the building, to the right side of the first turret you can disable. With this key you can open a secret room on the left side, at the end of the cargo hold of the hovercraft. Inside you will find a weapon crate with a weapon you have to dismantle for the first part.

Parts 2-4: From each of the 3 end bosses of the 3 Invaded Strongholds (they are on a weekly rotation and you can only do one per week).

Blueprint Guide

After you've received all 4 parts for the Nemesis sniper rifle (you get the last one from the invaded "District Union Arena" endboss) you can get the blueprint for the sniper from "Puck", the first boss in the invaded "Grand Washington Hotel" Mission.