I'd love to know how they have hooked VFS with WAL to monitor changes. The SQLite's WAL layer deals with page numbers where as VFS deals with file and byte offs

I'd love to know how they have hooked VFS with WAL to monitor changes. The SQLite's WAL layer deals with page numbers where as VFS deals with file and byte offsets. I am curious to understand how they mapped it, how they get new writes to the WAL and read from the WAL.