well, when you fetching a list of elements from the Dom you have two methods:
both methods will return (Simi-array of node elements), but not same array;
to take closer look lets add new element to this list and see what happens:
with the other methodgetElementsByTagName() will return a(HTML collection ) a LIVE-LIST got an updated with elements you added.
one more tip:
still we using the querySelector methods for flexibility we have when we select elements, and maybe can be better for performance when you select elements only on html and remember you still have that live references to the DOM so you can easy change the value of this reference(pointer) easy by textContent() method.