XMind does not support file synchronization itself. You need to use third-party cloud services, such as iCloud Drive, Dropbox or OneDrive to sync your files between different devices.
Here is an example to sync the files by iCloud Drive.
1. Upload your desktop mind maps to iCloud,
2. Launch XMind for iOS,
3. Enter Browser > Location,
4. Tap iCloud Drive,
5. Find the maps you uploaded from the desktop.