How to alter the fields in the productinfo (cart)

Hello Balbooa Team,

I have a very specific question.
For one customer, it is necessary to save a text in addition to the product options.

For example, there is product A with variant Z. Now I want a field to appear only when product A and variant Z are combined, in which I can enter a string of text. If product A and variant Y are selected, this field is hidden.

I have already implemented this part of the logic with CSS/JS.

My problem now is that I cannot transfer the text to the backend. Even if I change the text from variant Y to variant Y + string text, it resets this value when the shopping cart is updated.

My question now is:
What is the best way to transfer this data, and in which files are the necessary steps located?

Thank you very much for your help!

Best regards,
Dominique