I think there´s a bit problem in the code, certainly in the multiplication function.
function multiplicarDosNumeros(numero1, numero2){
return document.querySelector(selector);
}
The return value is wrong, it should be something like this:
function multiplicarDosNumeros(numero1, numero2){
resultado.textContent = numero1.value * numero2.value;
return resultado.textContent;
}
It´s just a little detail but needs to be corrected. ¯_(ツ)_/¯