It might be easier to start with a comparison of que and quel(le):

Que is an interrogative pronoun. It is never followed by a noun. It is often replaced by the phrase "qu’est-ce que" in spoken French. For example:

Que manges-tu ? Qu’est-ce que tu manges ?

Quel(le) is an interrogative adjective, so it is followed by a noun (except in one case – see below):

Quel film regardes-tu ?

BUT quel(le) (and not que) is used without a noun if the verb is "être":

Quel est ton nom ?

Quelle est votre question ?

Quels sont ces bruits ?

When it is in a question:
It may help to think of “qu’est ce que” as what, where you really don’t know the answer , and “quel” (quelle) as “which one”, where you know you are asking about something, but you only know a part: you are asking about a book, an age, a car, and you need an answer specific to that: which book, which car
Qu’est ce que tu as acheté? What did you buy? You are asking what. (It could be a kilo of tomato, a book, or a hat, you don’t know)
But:
Quel livre as tu acheté? Which book did you buy?
Quel chapeau as tu choisi? What hat did you pick?
In the second one, you have part of an answer as in, you know it’s a book, you are asking which one.

In english it is easier to swap what and which in many questions, what? What book?

So of you think of an expression like “quel age as tu?” You are asking a number, you already know a number is coming, you don’t know which one. So the idiom translate as “which age (number of years) do you have”.

“Qu’est ce que” is almost always part of a question.
“Quel” is trickier: it is used a lot in exclamatory sentences:
“Quel idiot!” (What an idiot).
Quelle bonne surprise! What a lovely surprise!
Quelle drôle d’idée! What a strange idea.
All of those would use “what”, not “which” in English… tricky!

It also can help to keep in mind that “qu’est ce que” is an old turn of phrase that just stuck and became part of the furniture in modern language. It seems bizarre, right? ” ” what- is -this- that”. Young kids would write “keskeu”!
“Aujourd’hui” is another weird but completely automatic one. Or “rez-de-chaussée” (ground floor)…

That doesnt cover all of it, but I hope it helps . Good luck