Are the updates on the update manager all by Ubuntu? What part of the update manager do you have sift through for, to avoid errors in one's system? Thanks.
By default, yes, the updates come from the Ubuntu repositories.
When you start layering in PPAs or commercial repos (as tends to happen over time), updates will be pulled from them, if they contain updated packages. The stability of these packages is not controlled by Canonical or Ubuntu.
What you have to "sift through for" completely depends on what you install and how you install it.