how do they do it ?

i've been reading about criminals and how highly sophisticated ones are able to get gift cards and reprogram the numbers in the mag stripe so that those number will allow them to go in store and spend money. just curious to know if anyone knows what sort of program they use and how is this even possible. there are probably millions of not hundreds of millions of different possibilities for that code they re-arranged to be a valid one, how do they know which ones are the valid ones or does a software do this for them ? i know about people programming emv chips in debit/credit cards which you would also have to be highly skilled in doing that other wise we would all be millionaires. just something im interested in chatting about. drop a line and lets hear your opinion.