Immigrant Ancestors is a project sponsored by Brigham Young University's Center for Family History and Genealogy. To create the database, student interns gather emigrant records from repositories throughout Europe, then bring them back to the project team at Brigham Young University (BYU). The contents are then extracted and verified by BYU student researchers before being added to the database.You can search a long list of archives from the website:
You can read a lot more about the project in an article by Jill N. Crandall and Charlotte N. Champenois entitled, "The Immigrant Ancestors Project: Gathering and Indexing 900,000 Names" published in the online journal MDPI Academic Open Access Publishing.
Here is the citation to the article and the link:
Crandell, Jill N.; Champenois, Charlotte N. 2018. "The Immigrant Ancestors Project: Gathering and Indexing 900,000 Names." Genealogy 2, no. 4: 51. https://www.mdpi.com/2313-5778/2/4/51