Why does UW need to extend the Schema of AD?
Please see the following KB article for a full explanation: https://helpdesk.lenovosoftware.com/portal/kb/articles/schema-modifications-to-the-directory-22-8-2017

How does UW modify the Schema? What are the necessary requirements?
The customer must create a domain service account for UW to use that has rights to read/write objects to the directory and modify the schema (only on initial install and during upgrades). Please see the following KB article explaining the service account: https://helpdesk.lenovosoftware.com/portal/kb/articles/creating-a-directory-service-account-22-8-2017

What modifications is UW making to the Schema?
During the initial installation, UW will add (not modify or remove) several auxiliary attributes and classes to the AD schema so that later, UW will be able to create new directory objects for storing our configuration. All of the specific attributes and classes that we add (along with the matching syntax) is described publicly on our KB here: https://helpdesk.lenovosoftware.com/portal/kb/articles/latest-schema-report-22-8-2017

What other modifications does UW make to Active Directory?
After the Schema has been updated, the installation wizard will ask to create a new container (OU) in the directory. This is where all of the necessary directory objects will be created that will serve as the base configuration for UW. Additionally, UW-specific information will be stored in the attributes of user and group objects.