AOSP 

The Android Open Source Project (AOSP) includes individuals working in a variety of roles. Google is responsible for Android product management and the engineering process for the core framework and platform; however, AOSP considers contributions from any source, not just Google.

Contributors make contributions to the AOSP source code. Contributors can be employees of Google or other companies, as well as individual developers with no company affiliation. There is no distinction between contributors; they all use the same tools (git, Repo, and Gerrit), follow the same code review process, are subject to the same requirements on code style, and so on.

GrapheneOS makes a great many changes and while like all other Android OS derivatives it is based on AOSP it is not AOSP. GrapheneOS has and continues to be one of the contributing projects to AOSP itself.