mdf $-$ Primary data file: This is the main file that contains the database schema and data. Every SQL Server database has one primary data file.

$.$ndf $-$ Secondary data files: These are optional and used when a database is spread across multiple files for performance or organizational reasons. They store user data just like the primary data file.

$.$ldf $-$ Log files: These files store the log information used to recover the database. There can be more than one log file, although typically one is sufficient.

In the group you've listed $($File$-03.$mdf$,$ File$-01.$ndf$,$ File$-02.$ndf$,$ File$-04.$ldf$,$ File$-05.$ldf$),$ there's a primary data file, secondary data files, and log files, making it a complete set for a SQL Server logical database.