Teams is best understood as a thin client over five backends:
| You see | It actually lives in |
|---|---|
| Chat messages | Teams chat service (Azure), journaled to Exchange Online mailboxes for compliance |
| Files tab in a channel | The team's SharePoint Online site (one site per team, one folder per channel) |
| Files in a 1:1 chat | The sender's OneDrive for Business (Microsoft Teams Chat Files folder) |
| Meetings & calendar | Exchange Online calendar (the Teams calendar is your mailbox calendar) |
| Membership & sign-in | Entra ID — a team is a Microsoft 365 Group; policies target Entra users |
| Voicemail | Exchange Online mailbox (transcribed by the voicemail service) |
Three consequences that surprise people:
- You cannot fully govern Teams from Teams. Retention for chat lives in Purview, file sharing behaviour lives in SharePoint admin, meeting recordings live in OneDrive/SPO.
- A "broken Teams feature" is often a broken dependency. No calendar tab? That's an Exchange licensing/mailbox problem, not a Teams one.
- Deleting a team deletes a Microsoft 365 Group — with everything the group owns (site, mailbox, planner). The group is the primary object; the team is a veneer over it.