Bootable USB with InstallOS.dmg?

Download the InstallOS.dmg. Open it and you will get InstallOS.pkg.

Double click on that and you will get an installation window, this does not install Sierra but converts the .pkg to

Install macOS Sierra.app which will be in your Application folder. Double-click on that to install Sierra.

If your mac came preinstalled with an OS newer than Sierra you cannot do the procedure to convert the .pkg to Install macOS Sierra.app. You need to do this on a mac that is capable or was capable of running Sierra.

What macOS version(s) are already installed on the Macs available to you now, and what macOS version do you want to install?

Find out which macOS your Mac is using - Apple Support

A .dmg file needs to be opened before you can use it to do anything, including using it to create a bootable installer.

How to create a bootable installer for macOS - Apple Support

vihaan__d wrote:

When I click on the InstallOS.pkg it opens an installation window, but does not convert the .pkg into an app - an installer app never shows up in my Application folder.

You need to let the .pkg "installer" start by clicking install so it extracts the app to the Applications folder. I mis-read this earlier thinking Catalina complained about not being compatible with this installer.

See steps 5 & 6 here:

https://support.apple.com/en-us/HT208202

vihaan__d wrote:

Thank you for your reply. Unfortunately, upon running the expand pkgutil command, three files are produced: Distribution.exe;

The "Distribution.exe" file should not exist if you downloaded the installer directly from Apple as that is a Windows executable file. Make sure to download the macOS installer files directly from Apple using the information from this Apple article:

https://support.apple.com/en-us/HT201372

It seems Apple has dropped the macOS 10.12 Sierra link from that article which most likely means Apple is in the process of fixing the broken installer. However, you can try to download macOS 10.12 Sierra from the link on this page (but this installer may still be broken when it comes to making a bootable USB installer):

https://support.apple.com/en-us/HT208202

vihaan__d wrote:

When I click on the InstallOS.pkg it opens an installation window, but does not convert the .pkg into an app - an installer app never shows up in my Application folder.

You can use a Terminal command to manually extract the installer. Start by typing the following leaving at least one space after "--expand ":

sudo  pkgutil  --expand   

then drag & drop the .pkg file onto the Terminal window to auto-fill the path to the .pkg file. Then continue typing on the same line:

/Applications

You should end up with the command looking something like (yours may look a bit different for the path to the .pkg file):

sudo  pkgutil  --expand  /Volumes/InstallMacOSX/InstallMacOSX.pkg   /Applications

If there are no errors and you don't see the El Capitan installer in the Applications folder, then you may need to add another folder to the end of the command like "/Applications/macos":

sudo  pkgutil  --expand  /Volumes/InstallMacOSX/InstallMacOSX.pkg   /Applications/macos

If you need to use the latter command, then after it completes drag the actual installer from "/Applications/macos" to the main "/Applications" folder.

Now you should be able to proceed with the "createinstallmedia" instructions in the original Apple article (assuming Apple fixed the Sierra installer).