It has to be stored out of the band, otherwise you would have chicken-and-egg problem: what's the shape of the array, when the info is in a file stored inside the array?
It's not stored in a file inside the array of course, but there are two types of out of band: next to the data, or completely disconnected. For example you only need to mount a single btrfs partition - in the header it already contains the information about other copies and will mount the whole raid setup as necessary. It doesn't matter if you move the drives somewhere else to a new system - mount one of them and it works.
On the other hand, if you move drives from a hardware raid and put in new drives, some (all?) controllers will read the raid config from memory and offer to build the same raid on the new drives. That's completely-out-of-band. Depending on the controller, even changing the order the disks are plugged in can give you weird results.
