I got reimbursed by Norton for an upgrade . However, they reimbursed it on my apple account. How can I transfer a great part of that sum to my bank account?

As advised, you would get a solution more quickly on the main Apple forum (which accommodates many different language speakers, including some posts in French) since the community here is primarily intended for questions in French.

What method was used to pay for the subscription? Refer to this article about refunds: Check the status of a refund for apps or content that you bought from Apple - Apple Support

If personal Apple Account balance was used, you cannot transfer that to your bank account, it can only be used with Apple.*

If a card of some kind was used to completely pay for the subscription, the refund should have been made back to the card directly. If it was not, you will need to contact a live (real person) Apple representative and ask if they will transfer this for you. It is not an official service and my understanding is they will only do this if no Apple Account funds were involved in the payment.

*From: Check your Apple ID balance - Check your Apple Account balance - Apple Support ➔

"About unused Apple ID balance

You can't redeem or return Apple Gift Cards, App Store & iTunes Gift Cards, or unused Apple Account balance for cash, except as required by law. If your jurisdiction allows for refund of redeemed gift balances, you can contact Apple Support." Read the support article for more details.

Hello johannes67,

Welcome to our French Apple Community Support site. We’re sorry being unable to assist you, but you’ll have  opportunity to receive quick and helpful reply by connecting to the English site. 

Log in here.  discussions.apple.com. 

Thanks you for your visit.