Due to network issues, downloading the runtime is not always smooth, so it is best to download it directly with support for resuming interrupted downloads.

1. Find the downloaded URL

Use console

1. Open Xcode, open preferences, go to the Components section.
2. Open the Console App, clear the console.
3. Go back to the Xcode preferences. Start the simulator download, then cancel it.
4. Now in the Console, you will see something about the cancellation with the download URL. ( Process is xcode and maybe search keyword download )
5. Copy the URL from the Console. Then download it by safari. ( or some other way which is authed )

Offical dowloaded from apple

1. Go to apple developer website and login.
2. You can find all Xcode dmg/xip files directly on https://developer.apple.com/download/all/ (requires Apple ID).
3. Find the simulator runtimes you want and download.

2. Download the DMG file

Now you can download it using Safari or another authorized method.

3. Add runtime

Finally, just run

xcrun simctl runtime add iOS_xxx_Simulator_Runtime.dmg