Technically they are not domains, they are subdomains of CO.CC – Free Domain name registration + Free DNS service. and while it is cheap to register them there is a $3 renewal fee for commercial use. $3 would be cheap for a domain but not so cheap for a subdomain. If you want cheap then register your own domain and you will have unlimited subdomains for free.
Now to your question of how they work for SEO. Google does not use domain-level signals. So, .co.cc subdomains work the same as any other domain or subdomain. All SEO signals are page level, and neither domains nor subdomains play a role in SEO, with the notable exception of .cc country codes. Using the co.cc domain may give you a slight advantage for ranking in the Australian territory of Cocos Islands, otherwise, it makes no difference.
