Microsoft Graph Calendars.Read allows the app to read events in user calendars.
For your app to access data in Microsoft Graph, the user or administrator must grant it the correct permissions via a consent process.
Application permissions are used by apps that run without a signed-in user present; for example, apps that run as background services or daemons. Application permissions can only be consented by an administrator.
References: https://docs.microsoft.com/en-us/azure/active-directory/develop/v2-permissions-and-consent
https://docs.microsoft.com/en-us/graph/permissions-reference

Explanation
Graphical user interface, text, application, email Description automatically generated

Oauth2 client_id client_secret
The authorization code flow begins with the client directing the user to the /authorize endpoint.
Box 1: token
Use the authorization code to request an access token.
Now that you've acquired an authorization code and have been granted permission by the user, you can redeem the code for an access token to the desired resource, by sending a POST request to the /token endpoint:
Box 2: authorization_code
Use the authorization code to request an access token.
Example:
// Line breaks for legibility only
POST /{tenant}/oauth2/token HTTP/1.1
Host: https://login.microsoftonline.com
Content-Type: application/x-www-form-urlencoded
grant_type=authorization_code
etc.
Note: At a high level, the entire authorization flow for an application looks a bit like this:
Diagram Description automatically generated

Reference: https://docs.microsoft.com/en-us/azure/active-directory/develop/v1-protocols-oauth-code

Explanation
There are currently three methods of app integration in Teams: Connectors, Bots, and Tabs. Tabs offer more extensive integration by allowing you to view entire third-party services within Microsoft Teams.
Reference: https://www.sherweb.com/blog/office-365/o365-microsoft-teams-apps/