This sounds more like however your OS handles opening the PDF mimetype(xdg-open,open,Invoke-Item) I'm assuming you're on windows. I think often times browsers will just be set to the default for previewing a PDF unless set otherwise. This is all just conjecture though as I don't use any of the tools you listed above and I'm not absolutely certain of how Windows/MacOS handles PDFs by default.

Twitter's handling of opening links in its own webview is a bit different, unless Slack, Teams, Confluence, Jira all open these browser instances within some sort of webview wrapper as well(I wouldn't think so). So its a little bit different

No, what they are talking about is that you click on a link to see a PDF in these web apps, and instead of serving up the PDF document itself, they serve up a page in their web app that embeds a PDF viewer.

I assume they are trying to be "helpful" but 99% of the time the user's browser can render the PDF more conveniently than the app's embedded viewer (not breaking scrolling and zooming etc.)