How do web services scan websites for viruses?

Question

Computer networks

Hello, it became interesting how exactly web services check the site for viruses? for example virustotal.com or others, for example 2ip.ru
Go to the site and scan the code of all pages by signatures? Where do they get signatures? It’s unlikely that they have a staff of employees who are looking for viruses, but there are no signatures in the public domain, after all, or are there?
Or are the methods completely different? How do they do it?
Are they downloading the contents of the site to themselves and being run by antiviruses?
Or how?

Answer 1

Right here you can ask,
support.kaspersky.ru/#s_tab1 Sign
up forum.kaspersky.com/index.php?showforum=7 Forum
Or https://www.virustotal.com/ru/faq/
2ip will also answer here. en/contact

Answer 2

there is such a thing as zero_day vulnerabilities that are found in js\java_aplet\flash\etc... all vulnerabilities need to be exploited somehow, and the implementation of the use of the code is already judged whether it is trying to use a known exploit.