Both Android and iOS use the WordPress.com API to connect with your WordPress.com username and manage your hosted sites. WordPress.com uses OAuth2 and requires a shared secret for application installs to be able to pair and authenticate. In order to connect to a WordPress.com account you will need to register for an account over at https://developer.wordpress.com. Once you have an app id and an app secret, follow the instructions here. The only WordPress.com account you will be able to connect to will be the one you used to register for a developer account. More details can be found here. You can connect to any self-hosted instances of WordPress without any additional configuration.