Overleaf V2 does not seem to have a way to get the BBL file for arxiv, how do I get it?
(Disclaimer: I'm a support personnel at Overleaf.)
Click on "Submit" and then choose e.g. "arXiv", there will be an option to download a
.zip with the
.bbl included. Once you click the button, it should show a "Please wait" message and take a while to build the zip. Then the button comes back, only this time it is a download link that will immediately download the zip archive.