With each update to Android, rather it's a large update or a small one, Google always publishes new factory images, rolls it out via OTA and uploads the code to AOSP. Seeing as Android is open sourced, the code is available for anyone to use. We also get a pretty lengthy changelog from Google's developers for Android 6.0.1, that a number of us don't know and likely don't care much about. Which you can read the Android 6.0.1 changelog here, for those that are interested. A lot of the changes are actually duplicates from the android-wear-5.1.1_r1 update.
There's not much new here. But one of the changes includes a big change to how ART works in Android. And should provide better performance. When installing an app, it should now do a bit less cleanup in the dex2oat converter. Which is nice to see. For example, with the Nexus 9, conversion went from 2.4 seconds down to about 0.1 seconds. That's a big change, and likely one that most people won't even notice, to be honest. There are also some changes for visual jank in some instances. This is thanks to change-6ef8578. In build MMB29M which is used for the Nexus 6P and Nexus Player, there are basically no changes. This update follows the MMB29K which was a bit bigger.
What's interesting is that the MMB29N build for the Nexus 6P actually goes from Android 6.0.1 to 6.0.0 and was built later on (but on the same day). The update is identical to the MMB29M update, although it is missing the new emojis. So other than the emojis the update is exactly the same, just missing the version number.
So the changelog didn't really tell us anything we didn't know already. The only addition was Android Runtime, which many of us won't see as an end user. We do know that the update brings Band 12 support for the Nexus 5X and Nexus 6P, as well as bring back the "until next alarm" option to Do Not Disturb, and lest we forget the 200 new emojis added in this release. The OTA isn't yet rolling out, but it should be on its way pretty soon.