You choose the store. The app writes and claims.
USB, object storage, NAS, or WebDAV. The same library can be claimed on another phone.
USB and portable-disk backup
iPhone picks a folder in Files. Android uses SAF. A library on the same disk can be claimed on the other phone.
Object storage: COS, OSS, S3
The keys are yours. The app keeps the profile on-device and writes originals into your bucket.
NAS and Samba
A LAN share as the backup disk. iOS needs Local Network permission. The SMB session starts on first I/O, not at save.
Claim a library on the other phone
The same bucket or USB library can be claimed on iOS and Android. That is the switch path, not export-then-AirDrop.
WebDAV
Write originals over HTTP to a WebDAV cloud drive or NAS. The address must be http:// or https://.