Introduction
The module’s integration lets you incorporate security features into your website, using Yubico.
Development
Core
Entity Definitions
The only entity definition, added via the module, is the YubikeyUserEntityDefinition. It adds the yubico-related attributes to the UserEntityDefinition. The inheritance goes further to the PrincipalEntityDefinition and the AbstractEntityDefinition:

The defined here property stores the yubikeyPublicId.
Business Services
The YubikeyAuthenticationProvider implements the Spring security AuthenticationProvider and the coming from the Spring beans factory InitializingBean:

The implementation checks the generated yubikey for the user. This guide shows how to generate a publicId for the customer and store it in your database.
The YubikeyUserDetails adds the yubikeyUser-related attributes to the UserDetailsDtoDefinition. The other extended interface is UserDetails, coming from Spring security:

For more details, please, refer to the Yubico documentation.