FAT and FFS are very different filesystems. FFS needs a superblock, backup superblocks, cylinder group maps with inode structures. FAT requires much more, such as the File Allocation Tables and the Cluster Map.
FFS
While there are many differences between UFS/FFS implementations,
this wiki page should help to understand the general structure of FFS/FFS2 on OpenBSD.
FAT
While this is not the only documentation availalble, here is
a key wiki page that may help you to understand the structure of a FAT filesystem.