Why do we say quest-ce qui se passe? Why don't we say quest-ce que se passe? Why do they use qui instead of que?

You use "qui" for the subject of a sentence and "que" for the direct object.

Qu'est-ce qui passe ? -- What is happening?

In this sentence, "what" is the subject in English and "qui" in French. "Que" can never be the subject.

https://french.kwiziq.com/revision/grammar/qu-est-ce-qui-verb-what-does

Hi Timothy,

It is a bit tricky to explain so I'm going to refer you here:-

https://www.lawlessfrench.com/grammar/indefinite-relative-pronouns-ce-que-ce-qui/

Hope it helps

Jim