In TFS 2017, we have a big project with multiple teams and areas within it, as suggested in this blog post.
I couldn't find out how to set a default Git repository for each of these teams, so the "Code" menu item at the top would point to a specific repository for each team.
No, we cannot achieve that. You have to navigate manually if you want to switch repository from one to another. It will not auto-switch, no such relationship between team and repository in TFS/VSTS.
As a workaround you can add the repositories reflect each team to Favorites, then you can switch to the corresponding repository conveniently from My Favorites in the account hub.