Edited to add
Based on the comments on this answer, it appears that Xcode updates via the system Software Update were delayed, appearing quite a bit after they became available for manual download from the developer portal.
It may even be that Apple fits them into times when their network usage was low, which could delay an update even if you clear out your old copy and reinstall the not-quite-up-to-date version. Perhaps the update would appear if you waited several days.
Previous answer
Xcode has never, ever updated for me through Software Update, and it's never updated for the dozen or so developers I work with. The Apple employees that I follow on Twitter -- guys like Jake Behrens, Michael Jurewitz (the dev tools evangelist), Chris Hanson, and Bill Bumgarner -- along with former Apple employees who are devs and other high-profile devs I follow, all tweet something along the lines of, "hey devs, time to visit the developer portal to download the new version of Xcode" when an update comes out.
Are you absolutely certain Xcode updates are arriving automatically for your coworkers? Have you seen it happen? Are you sure they're not pulling your leg? I've certainly never heard of it and consistently hear the opposite, so it sure seems unlikely to me.
The only automatic Xcode updating I'm aware of for Xcode 4, and only then if it was purchased through the Mac App Store and not downloaded from the portal, and only then if you launch the App Store and click on the update, which really isn't very automatic.
...enables you to create a recovery USB. While this recovery USB does not contain the installer image, it is able to install Mountain Lion (see 3rd paragraph).
You can download the official software from Apple here. Note that you can create the recovery USB only if you have a Recovery HD partition on your Mac. You can verify this in the Terminal using: diskutil list.
The Recovery Disk Assistant requires just 1GB because is does not store an OS X installer image (usually named InstallESD.dmg). Instead it will download the latest version of Mountain Lion right before the installation - if the recovery USB has been created in Mountain Lion.
You are still able to use the recovery USB without internet access if you just want to open Disk Utility from it for example.
Best Answer
You can get the dmg file from here:
https://support.apple.com/downloads/macosWhile downloading the dmg please check the system requirement. Because you will need a combo dmg file if you are upgrading from say 10.11 to 10.11.6
On detail page of the dmg there is a System Requirements button at the bottom, please check the system requirement from there.
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Source: Ask Different