I’m sorry but I like international law and consortiums exclusion right to ownership generally, so I would like that sort of answer to: How does copyright over online content that is not behind paywalled content work? Do people get some ability to share paywalled content?
Copyright law is about what you are legally allowed to do. What you are physically capable of doing is totally unrelated.
The fact that you can access something without anything stopping you or trying to stop you or without doing anything that would be considered "hacking" mean that you have the right to access the data.
But that doesn't give you any right whatsoever to make copies yourself and give them to others. You are most likely allowed to tell others where you found the data, and they will then find themselves in the same situation where they can download, for basically the same result, but that doesn't mean you are allowed to make copies.
PS. With technology being what it is, it might be for example that you need to be logged in with a stackexchange account to access some information. And that can be handled automatically by your computer - so it looks to you as if there were no measures to stop anyone from copying, but they are actually present. You can download something, but I can't. Now it's much more obvious that you can't give me a copy because the copyright holder didn't want me to have a copy. So the absence of a paywall that is visible to you doesn't mean there isn't one.
A more common example: I have the Netflix app on my phone and it looks like you can access anything without any protection. But that is only the case because the app has my username and password stored and I pay every month. You can’t access the same movies on your phone without paying, and you can’t access them on my phone because my phone will be locked.