You can have several .csv files with differing lengths, however, there needs to be a way to create a relationship between the files. What you are describing is a simple database, think of each .csv file as a table. How databases make relationships to other tables is through the use of keys. Unique keys are assigned to each new entry to the main table. These keys can be used to cross reference the user to other tables or .csv files.
The example below demonstrates a simple database using keys to tie information together.
Main.csv file
Key, Name
01, Bob
02, Jill
03, Sam
04, Mike
BMW.csv file
Key, Model
02, i8
Ford.csv File
Key, Model
03, Fusion
04, Focus
Fiat.csv File
Key, Model
01, 124 Spider
Using the example database, we can create a list of people and what type of car they drive by matching the keys.
Bob, Fiat 124 Spider
Jill, BMW i8
Sam, Ford Fusion
Mike, Ford Focus
Regards.