I think technically, legally, it is the opposite. Works are automatically copyrighted upon creation, so without a license, by default it is not free to use. In practice, who cares, that’s why they put it on GitHub, just use it

This is correct. The work is "all rights reserved" unless explicitly stated otherwise.

This has even been applied to projects that began with no license, but added one later on. An original author on some open source software can come back years later and claim copyright over the first few commits, when no license was present. That's why it is important to START your project with a LICENSE, and why it is a default option when creating a repository on GitHub.

The second paragraph here is a bit surprising. Do you have an example you can point to?

I think this would only apply in a situation where the original author stepped away from an unlicensed project that got forked or taken over, then the people forking it added an OS license (the original author could come back and license the original work under whatever terms they want)

If the original author added the license at some point in the future it would apply to the entire codebase at the time they added it, which they presumably hold the copyright for, so I don't think they could make the argument that some chunk of the codebase which existed at the time of licensing falls under a different license.

I suppose if an unlicensed project gets an early outside contributor, and then the original author adds a license, the outside contributor could claim they didn't consent to the license change (this is why many professional OS projects require contributors to sign a CLA)... but in practice I'm not sure how this would work, since this would mean the outside contributor forked and modified the unlicensed project, so they would have been potentially violating copyright in the first place

Is that universal or just a USA thing?

There are almost certainly going to be some exceptions, but copyright law is largely consistent in the broad brush strokes at least, among a large portion of the world's nations.

See:

https://copyrightresource.uw.edu/copyright-law/international...

It's a Berne Convention thing.

https://en.wikipedia.org/wiki/Berne_Convention

Not just the Berne Convention. TRIPS brought automatic copyright to WTO members as well, so it’s true of almost every country in the world.

TIL. There are apparently more Berne Convention signatories than WTO members, though, so I guess the former still has more reach, but I assume the intersection isn't empty, so of course TRIPS must have had some impact as well.