How do I mount an existing multidisk btrfs using kickstart?

How do I specify partitions for a multi-disk btrfs layout.
I have two exisitng volumes that are already labeled, sda is test-storage, with sdb and sdc being test-library.
I was able to mount sda with
part btrfs.01 --fstype="btrfs" --noformat --onpart=sda1
while the autogen used
part btrfs.01 --fstype="btrfs" --noformat --label=test-storage
I believe this is relevant because in the kickstart guide, it says:

Specifies that the partition should not be formatted, for use with the --onpart command.

So obviously there’s at least some documentation missing.
My issue is with disks sdb and sdc.

the autogen kickstart didn’t use part commands, so now I’m wondering how it figured out the device identifiers
btrfs /mnt/.library --noformat --useexisiting --label=test-library btrfs.72 btrfs.139
I found an older RHEL 7 article that stated

--onpart= or --usepart= - Uses existing blank device and format it to the new specified type. For example:
partition /home --onpart=hda1
puts /home on /dev/hda1.
These options can also add a partition to a logical volume. For example:
partition pv.1 --onpart=hda2
The device must already exist on the system; the --onpart option will not create it.
It is also possible to specify an entire drive, rather than a partition, in which case Anaconda will format and use the drive without creating a partition table. Note, however, that installation of GRUB2 is not supported on a device formatted in this way, and must be placed on a drive with a partition table.

So I tried below specifying the whole disks with no luck

part btrfs.21 --fstype="btrfs" --noformat --onpart=sdb
part btrfs.42 --fstype="btrfs" --noformat --onpart=sdc
btrfs /mnt/.library --useexisting --label=test-library btrfs.21 btrfs.42