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 upgrade macOS or reinstall macOS, but it can be useful when you want to install on multiple computers without downloading the installer each time.

Macos App Store

What you need to create a bootable installer

A USB flash drive or other secondary volume, formatted as Mac OS Extended, with at least 12GB of available storage
A downloaded installer for macOS Big Sur, Catalina, Mojave, High Sierra, or El Capitan

Download macOS

Find the appropriate download link in the upgrade instructions for each macOS version: macOS Catalina, macOS Mojave, or macOS High Sierra Installers for each of these macOS versions download directly to your Applications folder as an app named Install macOS Catalina, Install macOS Mojave, or Install macOS High Sierra. Create even more powerful Mac versions of your iPad apps. Apps built with Mac Catalyst now take on the new look of macOS Big Sur and help you better define the look and behavior of your apps. You can choose to turn off automatic scaling of iPad controls and layout, allowing you to precisely place every pixel on the screen.

Download: macOS Big Sur, macOS Catalina, macOS Mojave, or macOS High Sierra
These download to your Applications folder as an app named Install macOS [version name]. If the installer opens after downloading, quit it without continuing installation. To get the correct installer, 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.
Download: OS X El Capitan
This downloads as a disk image named InstallMacOSX.dmg. On a Mac that is compatible with El Capitan, open the disk image and run the installer within, named InstallMacOSX.pkg. It installs an app named Install OS X El Capitan into your Applications folder. You will create the bootable installer from this app, not from the disk image or .pkg installer.

Use the 'createinstallmedia' command in Terminal

Connect the USB flash drive or other volume that you're using for the bootable installer.
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 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, replace MyVolume in these commands with the name of your volume.

Big Sur:*

Catalina:*

Mojave:*

High Sierra:*

El Capitan:

* If your Mac is using macOS Sierra or earlier, include the --applicationpath argument and installer path, similar to the way this is done in the command for El Capitan.

After typing the command:

Press Return to enter the command.
When prompted, type your administrator password and press Return again. Terminal doesn't show any characters as you type your password.
When prompted, type Y to confirm that you want to erase the volume, then press Return. Terminal shows the progress as the bootable installer is created.
When Terminal says that it's done, the volume will have the same name as the installer you downloaded, such as Install macOS Catalina. You can now quit Terminal and eject the volume.

Use the bootable installer

After creating the bootable installer, follow these steps to use it:

Plug the bootable installer into a Mac that is connected to the internet and compatible with the version of macOS you're installing.
Press and hold the Option (Alt) ⌥ key immediately after turning on or restarting your Mac.
Release the Option key when you see a dark screen showing your bootable volumes.
If you can't start up from the bootable installer, make sure that the External Boot setting in Startup Security Utility is set to allow booting from external media.
Choose your language, if prompted.
Select Install macOS (or Install OS X) from the Utilities window, then click Continue and follow the onscreen instructions.

Learn more

For more information about the createinstallmedia command and the arguments that you can use with it, make sure that the macOS installer is in your Applications folder, then enter the appropriate path in Terminal:

Macos App Store

Big Sur: /Applications/Install macOS Big Sur.app/Contents/Resources/createinstallmedia
Catalina: /Applications/Install macOS Catalina.app/Contents/Resources/createinstallmedia
Mojave: /Applications/Install macOS Mojave.app/Contents/Resources/createinstallmedia
High Sierra: /Applications/Install macOS High Sierra.app/Contents/Resources/createinstallmedia
El Capitan: /Applications/Install OS X El Capitan.app/Contents/Resources/createinstallmedia

How To Create Macos Apps