Every team in MS Teams has a SharePoint Site Collection associated on the background. In the Teams tab you can see a SharePoint page with a ShortPoint web part added to it. The content of the tab is saved on the SharePoint site associated with that team.
Note: If you open the Site Pages of the SharePoint site, you won't see the pages from Teams tabs in there. These pages are system pages and can only be opened by direct URL, but still no contents will be visible there.