"Official" Android is a bit different. Updates are rarely sideloaded (going to a download site for the update) like Symbian, Mae, Mee, and Sail. Android's update system is probably the worst method they could have gone with, Apple's being only mildly better. Google's Android official update channel goes like this: Google makes the software patch and submits it to the manufacturer for approval/alteration. The manufacturer will then decide if it's worth them applying the patch to their custom build where they make it run for their hardware and apply their brand of bloatware and launchers (HTC Sense, Go Launcher, Samsung's S-launcher, etc). This update is then submitted to each Wireless Carrier that offers service for the phone model. Each carrier then decides whether to push the patch to the end user or not, applying their own additional bloatware apps to the package in the case of the former.
If a high level user goes the route of loading in custom Android ROMs, then in that case Android updates can be sideloaded. In this case, the updates are not provided by Google, it's provided by the maintainer of the ROM. Also, if the user decides to go with the custom ROM, they do so with the knowledge that any warranties on the phone have become Null and Void... and many insurance plans provided by the carriers or retailers will not cover a phone unless it has an official (Google's) version of Android on it that either came with the phone or was legitimately pushed to the phone by the carrier itself. This lends itself to the inevitable problem of if the phone is dropped and the screen is cracked and no longer turns on, but the phone's hardware can still be read, if there's a custom ROM on it the insurance can still refuse payout... meaning that the customer now has the option of paying full price for a new phone (minimum $200 for a "dumb" phone, minimum $500 for a smartphone), paying out the early termination fee (anywhere from $150 - $300, depending on remaining time in contract), paying the monthly service until such time as they can use the upgrade for a reduced price on a new phone, or if they were smarter, take their last phone out of the mothballs and use it until the upgrade period comes by again.
Being that I'm currently with Sprint, I've confirmed this where if I root my phone then crack the screen and want to file a claim on insurance, I have to be able to flash the stock ROM back onto the phone before submitting it for Claim review...which during that time I'll be going to my backup phone. Thankfully, ODIN (Samsung's flash tool) will work so long as the phone is able to power up and enter recovery, even if I can't see it on the Android's screen.