Welcome to the forum and massive congratulations on your new campervan! Roof corrosion is a long standing and much talked about issue with the aluminium pop top on the California. There are thousands of posts on this forum including those linked by
@clarinetbcn. It is an intimidating and depressing read but the good news is, you have an excellent campervan, the roof corrosion is aesthetic and it can be repaired under warranty. It is an acknowledged problem by VW. Before going down that depressing road. My advice is to enjoy your new campervan this summer and deal with the roof when you don’t have time to go camping later in the year. Good luck!