ASP.NET Zero template uses EntityFramework Core code-first and migrations. PhoneBookDbContext (YourProjectDbContext for your project) defines the DbContext class. Migrations folder contains EF migrations.
PhoneBookRepositoryBase class is the base class for your custom repositories. See Entity Framework Integration documentation for more.
You can use Package Manager Console to add new migrations and update your database as you normally do. See EF Core's documentation for details.