Some of my iTunes library music disappear from my iPhone

If they were synced from a computer, they can't just magically disappear.

What part of iCloud is on exactly?

The only relevant iCloud option for Music is the iCloud Music switch in the Music app settings in the Settings app. Which is only visible and available if you subscribe to Apple Music.

But you can always just sync your iPhone to your computer again to get all the music back on it if that was the case originally.

click here ➜ Sync content between your Mac and iPhone, iPad, or iPod touch - Apple Support

Impossible to know.

How did your music originally get on your iPhone?

Did you sync it from a computer using iTunes or Finder?

Where you subscribed to Apple music and recently cancelled it?

Did you turn off iCloud Music library in settings?

Did something happen recently that may have changed the music?