S’il t’arrivait quoi que ce soit, je ne saurais pas quoi faire.

With « arriver », the focus is on whether something happens or not. And it’s a one-off, happening rather quickly if it does, like an accident.

Tout s’est passé comme elle l’avait prédit.

Tout s’est déroulé comme elle l’avait prédit.

With « se passer » or « se dérouler », on the other hand, you can express how something happens. Also, it continues for a certain period of time rather than being over in an instant. Everything she had predicted came true one by one.

When something happens generally, it’s se passer, if you do not use an indirect object.

When something just happens: it’s se passer.

What happened last night? Qu’est-ce qui s’est passé hier soir?

Compared to:

What happened to him: An indirect object.

Qu’est-ce qu’il lui arrive?
What’s happening to him?

Qu’est-ce qu’il lui est arrivé hier soir?
What happened to him last night?

[I use the qu’est-ce here which is more usual in speech)

Now, if you want to say: What happened to him last night with se passer, you can do it like this:

Qu’est-ce que c’était passé avec lui hier soir?

But that is a bit different: What happened with him last night?