Pre-print Databases:
Wikipedia List of Pre-printt Repositories: https://en.wikipedia.org/wiki/List_of_preprint_repositories
ASAPbio (Accelerating Science and Publication in biology) list of 65 pre-print repositories, and tell who indexes them. Google Scholar indexes most of them. There is no repository indexed by someone that is not also indexed by Google Scholar. There are, however, a few pre-print repositories that are not indexed by anyone. https://asapbio.org/preprint-servers
Pre-print Wikipedia Category "Eprint Archives": https://en.wikipedia.org/wiki/Category:Eprint_archives
Preprint Repositories on the Wikipedia list, but not on the more extensive ASAPBio list:
APSA Pre-prints: https://en.wikipedia.org/w/index.php?title=APSA_Preprints&action=edit&redlink=1
Bellstein Archives: https://en.wikipedia.org/w/index.php?title=Beilstein_Archives&action=edit&redlink=1