Who is Fuchs? She is Raffensperger's Chief of Staff and has a very checkered history of political activism. Her hatred of Trump can be described as obsessive, and she was in Florida when she recorded the call in question. Why is that a problem? Because Florida is a two-party consent state.
In other words, she broke the law because Trump did not give his consent to be recorded. In fact, according to Isikoff and his co-author, she didn't have Raffensperger's permission to record the call either. After the conversation concluded, Fuchs immediately leaked a copy of it to the Washington Post, and the rest is history, including Willis' use of it in her case.
To put a finer point on how corrupt this was, Fuchs was supposed to be a star witness for the January 6th committee, but Liz Cheney and others agreed to not call her to publicly testify to shield her from possible legal exposure. Raffensperger himself also played a big role in protecting her because it had become obvious Fuchs had committed a crime.