Groups don’t have owners.

Your first command, sudo usermod -g www-data ftpuser made ftpuser’s default group into www-data. If you ran sudo usermod -g www-data root, you would, in kind, make the root user’s group be www-data. That’s not a good idea.

Groups do not have owners. Files and directories (folders) have owners. Users belong to groups, groups have members and every user has a default group, which is the group that gets applied to the POSIX permissions of any files or directories that user makes.