List of helpful sources for determining the copyright owner (holder):
An excellent overview on this topic can be found in Georgia Harper's "Who Owns What?" and its subpage "Sorting Through Ownership" in Copyright Crash Course, University of Texas Libraries LibGuides.
Digital Media Law Project, Works Owned by One or More Creators, (last updated 2014).
University of Michigan Library, Identifying Copyright Holders, in Obtaining Copyright Permissions (last updated 4/16/2019).
Useful information:
Digital Media Law Project, Copyright Licenses and Transfers (last updated 2014).