It doesn't do anything and was my attempt to give a simple example that is somewhat obvious while glossing over the complexity

    let arr = [{a: 1, b: ["a", "b"]}, {a: 9, b: ["a","c"]}];
    let b = "!"

    for(let e of arr) e = b;
    console.log(arr)

[{a: 1, b: ["a", "b"]}, {a: 9, b: ["a","c"]}] (unmodified)

    for(let e of arr) e.a = 2;
    console.log(arr)

[{a: 2, b: ["a", "b"]}, {a: 2, b: ["a","c"]}] (modified)

    for(let e of arr) {let copy = {...e}; copy.a = 4;}
    console.log(arr)
    

[{a: 2, b: ["a", "b"]}, {a: 2, b: ["a","c"]}] (unmodified)

    for(let e of arr) {let copy = {...e}; copy.b[0] = "!";}
    console.log(arr)
    

[{a: 2, b: ["!", "b"]}, {a: 2, b: ["!","c"]}] (modified)

The most frustrating thing about all of this is that the best way to make a deep copy to avoid all unwanted modification is JSON.parse(JSON.stringify(arr))

Providing It was syntactically correct the first call would assign e to a variable b for each iteration ( pretty pointless )

The arr.map providing you had a variable on the left side would output the result of each iteration into said array ( so an array containing all b values - however I guess you meant arr.map(e => e)

Nope. The first does not do anything. The second makes a new array of length 'arr.length' with 'b' in every cell

No, it's broken code and doesn't effectively do anything.