How can I download an older version of OS X/macOS?

Sometimes it's hard to search for older / previous versions of macOS & OS X on the App Store or Apple web site since they change links to promote the newest shipping version. *This is an attempt to provide the canonical QA for this issue, as per the Meta QA - https://apple.meta.stackexchange.com/questions/2418/where-is-the-list-of-canonical-questions-stored-for-ask-different I expect it to be modified over time to hopefully become a complete information resource.* How can I get any older version of a Mac OS to be able to run virtually (as a VM) or to reinstall a specific previous version on my Mac?