Green Open Access, also known as self-archiving, allows authors to make their work open access by depositing their author accepted manuscript (AAM) into an institutional or subject repository. There is no additional fee required for Green Open Access. The AAM is the version of your work that has been referred and accepted, but before the publisher has applied typesetting and branding. Many publishers allow you to share your AAM in these repositories, often with an embargo on access and a restriction on how much you can share, for example, one chapter of a monograph. The published final version will usually remain behind a subscription paywall by the journal, but the AAM will be available from the repository.
Digital Repositories
An institutional repository is an open access electronic archive of outputs written by authors from a particular institution. It is used to collect, preserve and disseminate digital versions of that output and is usually managed by the library or research office at that institution.
A subject or disciplinary repository is a digital collection of research outputs in a particular discipline area. They serve the needs of a particular knowledge community and can include both peer reviewed and non-peer reviewed publications.
Directory of Open Access Repositories (OpenDOAR): https://v2.sherpa.ac.uk/opendoar/
CORE: https://core.ac.uk/
COAR for a directory of preprint servers: doapr.coar-repositories.org/repositories/
Other repositories/pre-print servers:
https://oad.simmons.edu/oadwiki/Disciplinary_repositories
Examples of pre-print servers:
Arxiv: https://arxiv.org/
SocArXiv: https://osf.io/preprints/socarxiv