When using OverDrive, you can search each Massachusetts library's system's collection, and use your library card to check out a title from any of them. In the Libby app, you can add other library systems as a library and search through them to check out or places holds on their titles. All holds and checkouts will show up in one spot on your account.
On the OverDrive website, there is a link to "partner libraries" at the top, which you can use to visit their collections and check out or place a hold on titles. In the OverDrive app, you will need to add each library you want to borrow from before you can check out from their collections.