Does modifying an access file copy affect original file? This might me a stupid question, but it is crucial for me to know, so please bear with my limited knowledge of Access 2002 application.

Let's say I have an original database file, let's call it original.mdb. And I have Ctrl-C + Ctrl-V'd this file to the same location, with the same settings and everything, let's call it original_back.mdb.

Now, I want to understand the structure of files in Access 2002. Since both files will be pointing to the same database, will modifying a backup file affect the original? I might need to modify database for testing purposes, but I don't want to mess with original DB.
No, modifying a backup file will not affect the original. Access databases are self-contained in the MDB file.

Your statement "Since both files will be pointing to the same database" is incorrect. While both files are identical at the time of copying, they are (for all intents and purposes) separate databases.

If you're pointing an Access application to an external data source (e.g. SQL Server), this does not hold true.