How to create a bootable installer for macOS Source: Apple Support
You can use an external drive or secondary volume as a startup disk from which to install the Mac operating system.
These advanced steps are primarily for system administrators and others who are familiar with the command line. You don't need a bootable installer to install macOS, but it can be useful when you want to install macOS on multiple computers without downloading the installer each time.
Download a macOS installer, such as macOS Mojave ormacOS High Sierra. To download macOS Mojave or High Sierra for this purpose, download from a Mac that is using macOS Sierra 10.12.5 or later, or El Capitan 10.11.6. Enterprise administrators, please download from Apple, not a locally hosted software-update server.
When the macOS installer opens, quit it without continuing installation.
Find the installer in your Applications folder as a single ”Install” file, such as Install macOS Mojave.
Use the 'createinstallmedia' command in Terminal
After downloading the installer, connect the USB flash drive or other volume you're using for the bootable installer. Make sure that it has at least 12GB of available storage and isformatted as Mac OS Extended.
Open Terminal, which is in the Utilities folder of your Applications folder.
Type or paste one of the following commands in Terminal. These assume that the installer is still in your Applications folder, and MyVolume is the name of the USB flash drive or other volume you're using. If it has a different name, replaceMyVolumeaccordingly.
A bootable installer doesn't download macOS from the Internet, but it does require the Internet to get information specific to your Mac model, such as firmware updates. If you need to connect to a Wi-Fi network, use the Wi-Fi menu in the menu bar.
Select Install macOS (or Install OS X) from the Utilities window, then click Continue and follow the onscreen instructions.
For more information about thecreateinstallmedia command and the arguments that you can use with it, make sure that the macOS installer is in your Applications folder, then enter this path in Terminal: