If I publish a preprint on ResearchGate that was accepted on SSRN, for those who find it on ResearchGate, does the non-commercial license from SSRN apply, or will it just be CC-BY as offered on ResearchGate?
If your preprint was accepted on SSRN with a non-commercial license, that license would apply to those accessing it through SSRN. However, if someone finds it on ResearchGate, they would typically be subject to ResearchGate's CC-BY license terms. It's essential to review the terms of both platforms to understand the licensing implications fully.
As far as I understand it correctly the extent of rights (and license) is determined by the publisher of the paper and not by RG. See for more details here https://help.researchgate.net/hc/en-us/articles/14846037644817-Copyright-and-ResearchGate it states for example “By uploading and publicly displaying your work on ResearchGate, you are making it publicly available to all users of ResearchGate – both to ResearchGate members using the site logged in, and people without an account who access it logged out. ResearchGate does not require you to relinquish any rights that you may have in your work. Nor do you transfer or assign copyright to us.”