The DBML file is not related to the database server at all. It’s a completely client side thing. It’s essentially a set of information about your tables in the database and how you’re going to map them to .NET objects.
Of course, you can drag a table from a database and have Visual Studio infer some information for you automatically, but changing the file will not affect the database. You can create a DBML file from scratch without any database too.
Internally, the DBML file is simply an XML file that’s fed into a custom tool by Visual Studio and generates .cs files representing the LINQ object model for your database from it.