APEX modules are open source components of AOSP. See https://android.googlesource.com/platform/packages/modules/. Those modules include a lot of other AOSP code beyond what's directly in packages/modules too.

Google began shipping Google builds of the APEX modules via the Play Store to work around non-Pixel devices not shipping the latest monthly, quarterly and yearly OS releases. For Google Mobile Services devices, many of the APEX modules are required to be the official Google builds from the Play Store. The changes to APEX modules are released as part of the quarterly and yearly AOSP releases.