How to reinstall MacOS with an external bootable disk
- Update 2022-10 > Apple now has a page with download to all versions in here https://support.apple.com/en-gb/HT211683 > "Apple recommends using the latest (newest) macOS that is compatible with your Mac"
How to reinstall MacOS with an external bootable disk installer for Mac operating system. macOS Catalina, macOS Mojave, or macOS High Sierra. You can use an external drive or secondary volume as a startup disk from which to install the Mac operating system.
This article was inspired after I helped a friend recover their Mac operating system and documents. Having an external drive with macOS help and did a quick install. 
After installing from an offline version, do run the Apple software update to get the latest updates and versions. You can also run from the command line sudo softwareupdate -ai.
The purpose of this post is to share the links from support.apple.com for the installation of the various Mac operating system versions.
How to create a bootable installer for macOS
How to upgrade to macOS Catalina
How to upgrade to macOS High Sierra
How to upgrade to macOS Mojave
- https://support.apple.com/en-gb/HT210190
How to upgrade to OS X El Capitan
https://support.apple.com/en-gb/HT206886
How to reinstall macOS from macOS Recovery
Always have a backup of your data. Use at your own responsibility and happy learning,
Happy learning