Apple - How do you find the UDID for an iOS device?

You can follow the official how-to from Apple KB.

The same way works for iPhone, iPod and iPad.

The easiest way is to plug your iDevice, open iTunes, click on your device on the left panel of iTunes then click on the serial number. Your UDID will appear :

An alternative for those who don't actually run iTunes (yes there are alternatives) is to open the System Profiler, access the USB section, find the iPhone on the list of connected devices under the USB device tree.

On the iPhone listing there will be an entry that says serial number. This is not to be confused with the serial number listed in the General -> About section of the iPhone settings. The serial in the System Profiler is equivalent to the UDID.

Note that on Lion, System Profiler has been renamed System Information and you can get the the USB information from the hardware portion of the System Report.

This approach requires only a macOS device, the iOS device and the device's power cord.

If you have the iOS paired to the macOS device through wi-fi, you don't need a power cord and can skip steps 1 and 2.

This approach was based on macOS Catalina v10.15.5 and may not apply for other versions of the OS.

Steps

1. Connect the iOS device to an USB port of your macOS device.
2. If the iOS device asks for permission to trust the macOS device, grant it (assuming it's a device you trust).
3. Open the Finder app.
4. Go to the iOS device in the Locations section of the sidebar. It should open the device's page.
5. Click on the info section bellow the iOS device name on the top of the page until it displays the UDID.

You can copy the UDID by right clicking on it.

Source