Kundankumar, you’ve already asked a question very like this one, and I’ve given you a reasonably comprehensive answer.
In the case of system values, they just are values, like values held in data areas. The system will set some of them, like QDATE and QMONTH, because their values change in a time or date dependent manner. And other system values, like the password retry limit, can be changed by an authorised user. Additionally, you can use an API to change some (but not all) system values. There is no guarantee, though, that the system itself uses those same APIs when IT changes the system values.
Could you let us know just why you want to know about the underlying code for program and system values? Maybe if we in this community knew what you wanted to do, we could help you to do it.