Hi Patrick
you can get tab details from below API .
GET https://graph.microsoft.com/v1.0/teams/{id}/channels/{id}/tabs/{id}?$expand=teamsApp
to add new tab
POST /teams/{team-id}/channels/{channel-id}/tabs
https://learn.microsoft.com/en-us/graph/api/channel-post-tabs?view=graph-rest-1.0&tabs=http