If you navigate to a Team using the browser you can see and add additional tabs to an individual channel as shown above.
Since the mobile Teams app launched it was not possible to view those additional tabs. The only option you could access was the Files tab.
However, as you see above, with the latest iteration of the Teams app (on iOS and Android) there is now a Tabs option when you go into a Team.
Once you do you will then see the Tabs as shown in the browser view.
As with all Office 365 mobile apps, make sure that you update them regularly to take advantage of all the latest features.
CIAOPS 