Game files
Nintendo DS has two screens; the lower screen accepts touch. A .nds game is not a Nintendo 3DS game.
Keep an original copy of your own game files. On iPhone or iPad, choose Add games and select files in Files; on Mac, use File → Import, Command-I or drag into the window. Read the result for each file: added, already in your library or rejected. Renaming an extension does not convert a game.
Controls and screens
PlayStation, Xbox, MFi, 8BitDo, and other standard Bluetooth controllers. Keyboard on iPad and Mac. No setup.
Save and continue
Save inside the game when it offers that option. A manual Relay state captures a moment in the emulated machine; Auto Resume helps you return after an interruption. Keep Relay open for sync to finish, then check the destination has the expected game and progress before continuing. States require compatible game content and emulator versions.
Two devices disagree? Relay keeps both saves and asks you. Nothing gets overwritten.
iCloud and Relay Sync
Your library, saves, and play history. Game-file sync is free and opt-in. It all lives in your private iCloud storage.
Keep your library, saves and play history together with a Relay Account. Relay Sync gives you 100 GB; Sync+ gives you 500 GB. Both include Relay Pro while your subscription is active. Game-file sync is your choice.
Staying with Apple? Your private iCloud sync remains free. Relay Sync is a separate, optional storage plan.
Relay for Android is coming. Relay Sync connects the Apple and Android ecosystems: when the Android app arrives, use the same Relay Account to carry your library and progress between them.