Where does SQL Server LocalDB create its files, and what are those files?

When you create or attach an SQL Server LocalDB instance, SQL Server places its files in your user profile by default.

The default path is:

C:\Users\\AppData\Local\Microsoft\Microsoft SQL Server Local DB\Instances\MSSQLLocalDB

Inside that folder you'll find (among others):

• Files "master.mdf" and "mastlog.ldf", these are the system database files for the LocalDB instance itself
• Temporary and error log files
• Folders for user databases you attach manually.

When you create a new database under LocalDB without a specific path, it stores the .MDF and .LDF files under your "AppData\Local" by default

To see where your database files are, query:

SELECT name, physical_name FROM sys.master_files;

If you want to keep them elsewhere, specify the file path explicitly:

CREATE DATABASE MyAppDb
ON (NAME = MyAppDb_Data, FILENAME = 'C:\Data\MyAppDb.mdf')
LOG ON (NAME = MyAppDb_Log, FILENAME = 'C:\Data\MyAppDb.ldf');

These tips should get you going to the right direction. Remember to backup your LocalDB databases as well!