Research about the Epic Games's non-documented offically, Graphql API and main API, help and information at the Discord Server.
Most of the documented information is from the offical website and taken from Network inside Developer tools, then taken into a python script which will parse them into different folders and seperate files for the GraphQL.
If you are willing to add anything, make sure to make a new pull request and I'll see what it contains and if it can be added into the documentation.
Ways to find requests:
It can be done in multiple ways such as, using Developer Tools recording each request and using a script or finding each request yourself, or you could download a old Fortnite Party Hub version which I have many and use a program like Fiddler to see each request.
If there is help needed, search throughout the documentation and find the related information you are needing, if something is missing and you want to contribute make sure to, fork, create your changes, and make a pull request explaining what you've contributed.
There is something that we should of added? Well then make a issue and tell us what we are missing that we should of added and I'll see if it can be added into the documentation.
How can I talk to you or other people that can help me?:
There's two great ways to communicate and chat about what you need. You can make a issue that can be easily accessed at the repository and get answers. You can join our Discord Server which will give you direct contact to me, and other people that can help you. I am unactive at the moment, apologies.