Anecdotally, when I get calendar events for random conference services, I'll get a join button in the event which sends me to that service. Ditto for Teams invites from my organization. But teams invites from other organizations don't have the join button, neither in Outlook nor in Teams itself. I have to open the event and click the link. If I do it in Teams, I'll have to click the expand button to see the link.

I don't have any conferencing app installed aside from Teams. I join the others in the browser.