Why does my code produce error "'int' object is not iterable"

Here is the leetcode question: [Richest customer wealth][1]

and here is my solution:

class Solution:
    def maximumWealth(self, accounts: list[list[int]]) -> int:
        res = []
        for wealth in accounts:
            res = (sum(wealth))
        return max(res)

and I got the error such as:

TypeError: 'int' object is not iterable
    return max(res)
Line 6 in maximumWealth (Solution.py)
    ret = Solution().maximumWealth(param_1)
Line 25 in _driver (Solution.py)
    _driver()
Line 36 in <module> (Solution.py)

I simply converted this code into code above but I don’t know why this works and the another doesn’t?

class Solution:
    def maximumWealth(self, accounts: list[list[int]]) -> int:
        # 리스트 내포 사용
        accounts = [sum(wealth) for wealth in accounts]
        return max(accounts)

>Solution :

You want to add your wealth summary to a list and not replace it. So instead of

    res = []
    for wealth in accounts:
        res = (sum(wealth))

you want last line to look like

        res.append(sum(wealth))