Domain Generation Algorithms(DGAs) are used in malware to generate a large number of domain names that can be used in communications to the malware’s command and control servers
For example, an infected computer could create thousands of domain names such as: www.(gibberish).com and would attempt to contact a portion of these with the purpose of receiving an update or commands. - Wikipedia
But my question is we need to buy and register a domain name before we want to use. Then how hacker can generate 10 Thousand of domain name ? and use them ?