Hey coolqwerty,
AUMv2 uses extension deployed by the Azure Guest Agent/Arc Connected Machine Agent, but this extension will not directly install the patches. Instead, it will make calls to the client side patch management system to do the updates. The update client will then be responsible to performing the assessment, or installing the patches. The Update Manager extension it is responsible for sending requests to the client side patch management system but the responsibility stops there. Once the assessment/patch is complete, the Update extension will send the data for storage in It will store the data in Azure Resource Graph(ARG).
This is something by design which you are not able to view the extension on azure portal they are marked as hidden system extensions in Azure, if you wish to remove the extension you would need to raise a ticket with support who would then help you remove the extension.