my problem solved.It's complicated. Let's us focus on this phenomenon.
I have multiple layers, and I use tested Id : 1,2,3,4 , so firebase think my data id structure suppose to be :1,2,3,4,5... But the respond array or to say every array must have an index with 0 ..
It returns a 0 with null, cause I don't have 0 ,firebase made for me.(Suddenly,I seems like feelling the love with AI. To be honest...)
However, the real id is not '1,2,3,4' , I randomly enter an id '123455'.....
Firebase turns to be confused and realizes that 'oh! ,you data structure should not be an array. Let's try an deeply nest object...'
Problem: when id = 1,2,3 , firebase make '0' for you

So how can I get the data return?
Because Javascript sometimes not reliable ... on typeof()..
const data = await res.json();
console.log(Object.prototype.toString.call(data));
if it's an array . it will show [object Array] , if it's an object . it will show [object Object]
You can write two ways to parse data...