See "Why is my publication already listed on ResearchGate?" in https://explore.researchgate.net/display/support/How+to+add+research to learn about the ways how publications are added. There are partnerships with Wiley and Springer, see https://help.researchgate.net/hc/en-us/sections/14292219962385-Research-Publications (at the end of the page). However, there seems to be no way for users to create connections to other platforms like Scopus.