Calculate function with optional operator argument
Use a function declaration function rechne2b(a, b, {operator}){ rather than a function expression let rechne2b = function(a, b, {operator}){
There is only a single optional argument so don't use the rest operator ...
The optional argument is passed in an object so you should use destructuring assignment  function rechne2b(a, b, {operator}) to extract the operator if given.
Avoid switch statements if you can by using a lookup (see example)

Rewrite
function rechne2b(a, b, {operator}) {
    const ops = {
        subtrahiere(a, b) { return a - b },
        multipliziere(a, b) { return a * b },
    };
    return ops[operator] ? ops[operator](a, b) : a + b;
}