I have always been curious as to how recharge (scratch) card numbers are converted to airtime when loaded. Same for electricity units and other similar cards. The experts in the field, kindly shed light.

Recharge cards are stored in a database by the network provider, whenever you try to load it checks the database for that card and if it exists, it checks if the status is used or available, if it is available then it checks the airtime value and allocated that amount to your sim

for nepa, every prepaid machine is distinct and has a distinct meter no that isnt shared with another meter. an algorithm is designed that does some manipulation using your meter no.
so lets say you want to load 2k, it uses your meter number as a variable to run a hidden algorithm/encryption to then create a string of numbers that your meter decrypts it to get the units allocated to you.
I think you meter then stores this number somehow in its memory so that it cannot be used again..

I'm not so sure but thats what I think goes on

If you are so curious to ask this question, why not consider a degree in Electrical/Computer Technology?