How to extract Software Update in macOS 10.14?
It seems to me the main goal here is that you need to regularly update multiple Macs and you'd like to do that without having to download them each time for each machine.

I see two potential options here:


Use content caching
Download from Apple's website manually


Content caching

According to Apple:


  Content caching is a macOS service that helps reduce Internet data
  usage and speed up software installation on Mac computers, iOS devices
  and Apple TV. Content caching speeds up downloading of software
  distributed by Apple and data that users store in iCloud by saving
  content that local Mac computers, iOS devices and Apple TV devices
  have already downloaded. The saved content is stored in a content
  cache on a Mac, and is available for other devices to retrieve without
  going out over the Internet.


Source: Apple - About content caching on Mac

In essence, you can enable Content Caching via the Sharing preferences pane in System Preferences to manage content caching on your Mac and select the items you want cached. Once cached, other Macs on your network can use this to perform updates from.

Refer to About content caching on Mac and Manage content caching on Mac on Apple's website for more info. 

Download updates manually

You can download all updates directly from Apple by pointing your browser to: https://support.apple.com/downloads

From there you'll see the most recent software updates available for download, and you can also use the buttons at top to browse downloads by product. There's also a search field you can use to conduct specific queries.

Downloads will be in the .dmg disk image format and you can then keep and use these as required on your multiple Macs.