Can someone explain this 8 note thing to me?

First ofd I’m out of my depth, I do NOT know music. Love it but never learned anything more than how to hold a beat. Can’t sing and the last instrument I played was a third grade recorder, not for lack of interest but lack of opportunity. But I digress. I’m just trying to properly frame the lack of knowledge I have coming here.

I also did a search online but I didn’t know what to call it and even the ai assistants I’d prefer not to use didn’t understand what it was I was trying to ask

As mentioned I can hold a newr I just cannot play music. And so I’m trying to help my son find the beat in songs, I have him start tapping on the quarters, then half, then whole and so in and doing it with different songs so he has to find the best and no matter what the song does be it vocally or drops the beat or whatever he had to hold the beat. And then we started counting the notes and I wan noticing that almost every song we listened to (with some exceptions) always chsnged after the 8th note. At lot of times it’d match the chorus of a song but even edm/techno or orchestral pieces seemed to follow that.

So this seemed like a music theory question more than just a music question but what is that? Is there a rule about 8 notes? Is it just a common buy not necessary practice or an I just finding patters where there aren’t any?