Bot

So there are millions of computer devices around the world. whenever the computer has the vulnerability infected with the computer. The vulnerability has the payload which has a job to infect the computer and turn it into Bot whenever computers to turn into Bot it has to connect to the Botmaster so there will be millions of computers around the world that they have been infected because of the vulnerabilities that they had.
The thing is the Botmaster tries to control each of the nodes as a Bot so basically, the Botmaster tells them to do what sense them the command to actually open a website sending Spam, distributing malware or infecting other machines. this is the way that the Botnet works.