The MS hardware requirements for Teams are here:
https://learn.microsoft.com/en-us/microsoftteams/hardware-requirements-for-the-teams-app
and says Teams supports the last three macOS versions, which today are: Big Sur (OS11); Monterey (OS 12); and Ventura (OS13). Your iMac is maxed out a 10.15 Catalina; you need at minimum a 2014 or newer iMac to use Teams per that article.
It says nothing about partial support like you seem to be experiencing. As Teams is a Microsoft product, I believe you should ask them as well. Their forums are here:
https://answers.microsoft.com/en-us/msoffice
Be sure to include "mac" in your search string to limit the search to Teams Mac topics. Same if you post a question—include "Mac" in the title. That makes it easier for other Mac users when searching for new posts.
I believe you will find the contributors there knowledgeable and very helpful, much more so than MS's frustrating Windows-centric support documents.