OK, as to the Pixel phones, I have a Pixel XL and a Pixel 2 XL. My carrier is Project Fi. Project Fi operates over 3 telecommunications companies, Sprint, T-Mobile, and U.S. Cellular. As a Project Fi customer using Pixel and Nexus phones, I have learned over time that if T-Mobile customers haven't received an OS update yet, for whatever unknown reason, I should not expect to receive it either.
Usually, my phones receive updates in a timely manner. However, this September was the exception that proved the rule. My Pixel 2 XL received the September update on the 10th. However, my Pixel XL did not receive it then, and still hadn't received it a week later. So, based on past experience, I had a look around and noticed that a lot of T-Mobile customers with the OG Pixel XL were complaining about not receiving the update. That told me the issue was not with my phone, and I just stopped being concerned and waited to hear that T-Mobile customers were getting the update on their Pixel XL phones. Sure enough, when they got it, my Pixel XL got it too... on September 27th! (By the way, Verizon customers began receiving the update on September 4th.)
Sure, Google controls the updating process. However, once the updates are available, it seems there must be some sort of "approval" or other process that involves the carriers, and that sometimes causes delays in the OTA process even if/when the update is available for sideloading. I don't claim to know why it happens that way, I just know from experience that it does.
https://forums.androidcentral.com/google-pixel-pixel-xl/907669-september-update.html
https://forums.androidcentral.com/showthread.php?t=911087&p=6346211&viewfull=1#post6346211