There’s no official Syncthing app for iPhone/iPad due to technical and security restrictions built into iOS.
See this older thread for details: Syncthing for iOS
Here’s a protocol compatible Syncthing client for iOS (freemium license):
Note that Möbius Sync cannot do background syncing per iOS restrictions: FAQ | Möbius Sync
Similarly, newer versions of macOS required Microsoft, Dropbox, Box and other cloud storage providers to overhaul their respective desktop sync clients to be compatible with Apple’s “File Provider” subsystem. In many ways the change is similar to how you can install Firefox, Chrome and other web browsers on iOS but they all must use the bundled WebKit rendering engine just like Safari.