What is the operating system written to, and what does nonvolatile mean?

Non-volatile memory (NVM) is a type of memory that retains stored data after the power is turned off. Unlike volatile memory, it does not require an electric charge to maintain the storage state. Only reading and writing data to non-volatile memory requires power.

you can only change it if your real name or personal information was in it. and if so you would need to contact them directly with a username change request.