Universitat Bielefield, Bielefield, Rhine-Westphalia, Germany.

BASE (Bielefeld Academic Search Engine) is one of the world's most voluminous search engines especially for academic web resources, for examplee: journal articles, preprints, digital collections, images / videos or research data. Try searching with BASE right now!  (www.base-search.net/)    Provides basic, advanced searching and browsing options along saving the searches you have performed.
What is different about BASE?  BASE facilitates effective and targeted searches and retrieves high quality, academically relevant results. Unlike search engines like Google or Bing, BASE searches the deep web as well. The content providers which are included in BASE are intellectually selected (by people from the BASE team) and reviewed. That's why data garbage and spam do not occur. Read more details about the project.

BASE FAQs: https://www.base-search.net/about/en/faq.php

The British Library's EThOS,  https://ethos.bl.uk/Home.do aims to provide:

• A national aggregated record of all doctoral theses awarded by UK Higher Education institutions
• Free access to the full text of as many theses as possible for use by all researchers to further their own research.

There are approximately 500,000 records relating to thesis awarded by over 120 institutions. Around 260,000 of these also provide access to the full text thesis, either via download from the EThOS database or via links to the institution’s own repository. Of the remaining 220,000 records dating back to at least 1800, three quarters are available to be ordered for scanning through the EThOS digitisation-on-demand facility. (There is a charge for this service.) Records are held for all UK PhD-awarding institutions, but they do not (yet) hold all records for all institutions. Every month around 3000 new records are added and an additional 2000 full text theses become accessible.

You must register and create a login to download documents and maintain a history of your searches  https://ethos.bl.uk/Logon.do   Basic search and advanced search options available.

 🇺🇦   make metadata, not war

CORE (Connecting Repositories) is a service provided by the Knowledge Media Institute {Wikidata] at the Open University in the United Kingdom.
It is an aggregator serving a global network of open-source repositories and journals.  It has "the world's largest collection of open access research papers.  https://core.ac.uk/

         Zenodo - Research. Shared.

Zenodo is a repository for research. Authors and researchers can upload material and make it private or public.  Zenodo is funded by CERN, the European Community and OpenAire.  All data and research is stored at CERN and made searchable by OpenAire.  Focusing on scientific research is does include materials from other disciplines.   https://zenodo.org/

Google Scholar

Google Scholar is a search engine for articles or case law.  You can search across scholarly articles from many disciplines. It has an advanced search function link on the left side of the page [click on the three bars] which allows you to refine your search. When you sign in you can save articles and materials you have found to "my library" for future use.
 

Google Books

Google Books allows you to narrow your search to just books. Google states that it is the world's most comprehensive index of full-text books.  One needs to remember that not all books have been digitized, but it is still a very valuable reseource.