Every package gets updated, when there is an update for it. Dependencies don't matter for that.

Only if a package depends on a specific version of another package, is there a difference. When the dependency would get updated to a version that doesn't satisfy the version requirement. Then an error gets thrown and nothing gets updated.

But the package maintainers for the official repos don't really let that happen. It's more of a problem with aur packages.

Are you talking about package dependencies that were pulled in after compiling a package from AUR ? As far as I know (I'm not much of an AUR user, except when I quickly want to test some software. Please correct me if I'm wrong) if you normally would run sudo pacman -Syu you can instead of that run sudo yay -Syu (or other AUR helpers that support this) which would update both Arch packages and these from AUR.

In my experience, it's idependently unless updating breaks a version dependency on another package. In that case pacman won't update anything until the issue is resolved. I think I've only ever had this happen with AUR packages.