I don't understand how permutations work

[nweaver]

function permutations(string) {
  if (string.length <= 1) {
    return [string];
  }
  
  let finalPermutations = permutations(string.substring(1)) // how does this work?
    .reduce((acc, p) => { 
      let charList = p.split(''); 
      for (let i = 0; i <= charList.length; i++) { // how does this work?
        let newPermutation = charList.slice(0, i) // how does this work?
                              .concat([string[0]]) // how does this work?
                              .concat(charList.slice(i)) // how does this work?
                              .join(''); // how does this work?
        if (!acc.includes(newPermutation)) { // how does this work?
          acc.push(newPermutation); // how does this work?
        } 
      }
      return acc;      
  },[]);
  return finalPermutations;
}

[ptola]

It takes the string without the first character. "abcd" → "bcd".

For every length ("", "b", "bc", "bcd") it appends the first character (a) of the string.
→ "A", "bA", "bcA", "bcdA"

Then it appends the rest. → "aBCD", "baCD", "bcaD", "bcda"

Recursively