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.)
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