Where to find the keychain stored by KeychainSwift

There's no user interface for this part of the keychain what what is included in the app (or apps) created by that specific developer.

The keychain area that is accessed by the iOS app is specific to that app (or group of apps by the same developer, if specified). It cannot be accessed by other apps, so no App Store app will allow you to show the contents of the keychains of other apps. Nor are the app specific keychains areas viewable through the Settings app or any other first party app.

As a developer, you can add such user interface to your own app - or for debugging purposes simply use the Xcode debugger or add prints to the console with information from the keychain.

The Keychain can in general be synchronized over iCloud with other devices - including macOS devices. This does however not happen automatically for third party apps. The app would need to explicitly asks for iCloud syncing using the kSecAttrSynchronizable attribute.

Even then, the macOS Keychain Access app does not allow you to view all the data stored in the keychain by a third party iOS app.