iPhone and iPad
RetroArch from the App Store has Cloud Sync. Use its WebDAV backend with SaveSync, and your iPhone shares saves with devices iCloud can't reach: Android handhelds, Steam Deck, and PCs.
https://savesync.fastwater.app/saves/, with the slash at the end, then enter the Username and Password from SaveSync.cloud_backups folder before it overwrites or deletes it.Besides starting RetroArch and closing a game, RetroArch on iPhone and iPad also syncs when you come back to the app. Close the game before switching to another device anyway, so the last save is uploaded.
RetroArch syncs saves by their path inside its saves folder. Two devices only share a save when the path matches, so check these settings in Settings, Saving on every device:
The game's file name matters too: Pokemon Emerald (USA).srm only matches a ROM with the same name. Rename the ROM on one device if they differ. Use the same core for a game on each device as well, since a core's saves sit in its own folder when sorting by core name is on.
RetroArch syncs when it starts and when you close a game, not while you play. Close the game before you put a device down, and let RetroArch start on the next one before you load the game there.
If two devices change the same save between syncs, RetroArch skips that file and says "Cloud Sync finished with conflicts". To keep the copy in the cloud, delete the save on the device and sync again (in Settings, Saving, Cloud Sync, Sync Now). SaveSync keeps the 20 newest versions of every save either way, so you can restore one from the dashboard.