There's also two different applications which are both "Outlook for Mac".

If you go into the "Outlook" menu in the app, there's a "Legacy Outlook" button, which relaunches outlook using a completely different binary. The two outlook implementations have different bugs and all sorts of different behaviour.

Outlook For Mac is free but "legacy outlook" requires a MS365 subscription for some reason.

Outlook is also not to be confused with Microsoft's "Web Outlook" client, available at outlook.live.com. It all seems totally insane.