e refers to element in the array of the current itteration and i the index of the element in the current itteration on each itteration we are substraction the index i from the input number n populating the array with decreasing numbers from first number n each time by 1

What's the 'e' needed for in the map callback? I tried without it and it doesn't work but not sure what that 'e' is actually doing

It's not necessary, this 0 is never used (e isn't either).

Is the 0 inside the fill method necessary? I just tried with fill() and it worked.

Btw, I think your clan name is wrong, the correct is #100Devs - leonnoel.com/twitch, you have to remove the http:// from yours.