How do I use AssetScan Lite?

AssetScan Lite and its companion apps, are designed to be used for aiding in managing large estates of iOS devices.

In order to use AssetScan Lite, you will need the following:

• An MDM that supports App Configurations.
• An Apple Business Manager (ABM) or Apple School Manager (ASM) instance
• Apple's Apps & Books (formally Volume Purchase Program)

Different MDM's will have their own mechanisms for integrating with Apps & Books and with deploying custom App Configurations.

For Jamf Pro you would utilise the following App Configuration with AssetScan Lite

<dict>
    <key>serialNum</key>
    <string>$SERIALNUMBER</string>
</dict>

Note: your MDM may reference the device serial number in a different way. Please check with the documentation for your MDM

Why isn't this easier?

A few years back it was. Unfortunately unscrupulous app developers used people's device serial numbers to track them across a variety of apps and locations in order to invade their privacy and track their actions. Apple took the decision to make the device serial number invisible to the application frameworks so that app developers could no longer find out the device serial number.
Fortunately your MDM can see this and can tell an application what this is.