In my understanding, some people have donated their academic login credentials, effectively giving Sci-hub access to their institution's subscriptions.
When somebody requests an article, Sci-hub will try and see if it already has it. If not, it will attempt to use some of the donated credentials to gain access, download it and store it for further download requests.
When somebody requests an article, Sci-hub will try and see if it already has it. If not, it will attempt to use some of the donated credentials to gain access, download it and store it for further download requests.