My grandfather worked on the Manhattan Project! I am not certain of what meaningful contributions, if any, my grandfather had nor how long he was apart of the project, but I am confident that he remained in NYC for the entirety of his work -- never in Los Alamos to my knowledge.

Nevertheless, I remember asking him what was it like to actually work on the project. He said that it was far less Hollywood-esque than many would imagine -- at least for him. He was just given math/engineering problems and was asked to solve them with no context. He never knew what he was truly working on, why he was working on these problems, etc.. The work was pretty isolating and contact was with others was pretty minimal. I do know that he met both Von Neumann and Oppenheimer on at least one occasion which is pretty awesome.

I wish I could find some records, but I do not even know where to look.

My grandfather also worked on it, as a technician in Los Alamos.

He had previously been working for a scientific supplies company in Chicago that was (unbeknownst to him) providing supplies to the Manhattan Project. Apparently his boss was aware of it, and when my grandfather's draft was called a letter from his boss convinced the draft board to assign him to Los Alamos instead. He was eventually able to get my grandmother, a secretary and typist, a job as a secretary in Los Alamos as well so that she could join him. She teased him the rest of their lives, because as the secretary to someone more important than a lowly technician, she had technically had a higher security clearance than he ever did!

The Atomic Heritage Foundation collects records about people who were affiliated with the Manhattan Project, as well as oral histories. Perhaps they have more information about your grandfather's work? See here: https://ahf.nuclearmuseum.org/ahf/bios/

If you want advice on archives, you should just write to one of the historians who are experts on the Manhattan Project. The preeminent expert on nuclear weapons history is Alex Wellerstein, who also has a very well-regarded blog that comes up on HN from time to time: https://blog.nuclearsecrecy.com/

My great-grandfather was part the team who selected the site for Oak Ridge (the only civilian, together with four military men - he was the TVA's chief mechanical engineer at the time), and he also selected the separation method to be used. The town of Oak Ridge even named a day after him when he retired.

>I wish I could find some records, but I do not even know where to look.

The National Atomic Testing Museum in Las Vegas had a room full of file cabinets full of records you could look through the last time I was there, that might be a start.

Maybe one of the national labs that currently works on stuff has public records?