Python. I'm asking you to ask me a rudimentary function question. (Related to self)

I'm currently studying Python 3, so I'd like to ask you a basic question.

I don't understand why one of the codes below is executed by receiving one of the parameters self from the sum and avg methods. I know that Python needs to explicitly implement self as the first parameter, but shouldn't the actual parameter be substituted after self?

I think I'm the only one who's confused about the easy part, so I'd really appreciate it if you could answer me!

class Calculator:
    def __init__(self, numberlist):
        self.numberlist = numberlist

    def sum(self):
        result = 0
        for num in self.numberlist:
            result += num
        return result

    def avg(self):
        total = self.sum()
        return total / len(self.numberlist)

cal1 = Calculator([1,2,3,4,5])
print(cal1.sum())
print(cal1.avg())