How does the code work on Netflix?

I don't have Netflix yet, but I want to buy it. How does this work? My plan is to go to my Rewe and get a, let's say, 15 euro Netflix card. If I then created the Acc and entered the code of 15 euro, but I get a subscription under 15 euro, what happens?

Will the remaining money be saved on the Acc?

Note: I just want to redeem the code. No other things!

Yes, the money is "saved" on the account. So if you get a subscription for 12 euro, you save 3 euro, so to speak

How to access Netflix's secret movie codes
https://de.vpnmentor.com/blog/wie-man-auf-die-geheimen-film-codes-von-netflix-zugreift/

The remaining credit remains stored and will then always be offset in the future.