How do Fortnite Leakers actually find out exactly what comes with each update and so how do they do it?

How do Fortnite Leakers actually find out exactly what comes with each update and so how do they do it?

By carefully examining the game code for new content.

I don't know how to get the game code.

They examine the game code for new files that were not previously there, so they usually find new skins and content unless they are encrypted, so that doesn't work.

One possibility is data mining.

Many hints like skins are already integrated in the game code before the update.
And you can filter them out.