How and where to save information from multiple lists?

Question

Dart

There is a .csv file (in the application path). While working with the file..., some lines from it will be destroyed and added to the other three files, depending on the conditions in the program. In the first one, for example, five new lines can be added; in the second, seven lines; in the third - ten.
How to organize work with these arrays so that after closing / opening the application, they retain their new length. That is, the uploaded file will decrease by (5+7+10=22) lines. The first will have 5 lines, the second - 7, the third -10.
As I can see. The csv file is being loaded. Converted to a list. Further, three more lists are created, and, as necessary, they are filled. There are four lists in total. This seems to be clear.
And how to save all these four arrays? As a separate csv file, as four csv files? Or in some other way?
In what area of ​​memory are such files usually saved? In assets or next to the source file? Or somewhere else?
And one more important point. How to organize "spontaneous" saving of new lists? After all, the user can exit the program at any time. And every time I don’t want to ask him whether to save the program or not. FINALLY SAVE.
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