Explanation

Step: With DPS (Device Provisioning Service) you can generate device credentials and configure the devices offline without registering the devices through IoT Central UI.
Connect devices that use SAS tokens without registering
1. Copy the IoT Central application's group primary key
2. Use the dps-keygen tool to generate the device SAS keys. Use the group primary key from the previous step. The device IDs must be lower-case:
dps-keygen -mk:<group primary key> -di:<device ID>
3. The OEM flashes each device with a device ID, a generated device SAS key, and the application ID scope value.
4. When you switch on a device, it first connects to DPS to retrieve its IoT Central registration information.
The device initially has a device status Unassociated on the Devices page and isn't assigned to a device template. On the Devices page, Migrate the device to the appropriate device template. Device provisioning is now complete, the device status is now Provisioned, and the device can start sending data.
On the Administration > Device connection page, the Auto approve option controls whether you need to manually approve the device before it can start sending data.
Reference:
https://docs.microsoft.com/en-us/azure/iot-central/core/concepts-get-connected