To transfer (share) data between multiple devices, use SQUARE ENIX BRIDGE.

If the account link to SQUARE ENIX MEMBERS is not yet completed:
First, open the BRIDGE data sharing page from the title screen or CAMP menu of the source device and complete the account link to back up the game data. Then proceed to the steps under 'If the account link to SQUARE ENIX MEMBERS is completed' below.

If the account link to SQUARE ENIX MEMBERS is completed:
On the destination device, open the account selection page from the 'Data Transfer' button on the title screen or the 'Data Link' option in the game menu, select the linked account, and complete the data transfer.
Note that executing the transfer process will restart the app.

Following these steps for data transfer (data sharing) will share not only the game data but also the gems (both paid and free) held by the account.
Sharing is possible between iOS and Android devices as well.

Please note that quick save data is stored on the device, and it will not be possible to resume from the quick save data after switching devices.