How to create a bootable OS X Snow Leopard USB drive? How can I achieve this from either macOS Catalina or Windows 10? I can't seem to find a guide online. I've downloaded Mac OS X Snow Leopard Install DVD from the Internet Archive. Keep in mind that this is an .iso file, not .dmg.
I don't need a full step-by-step guide, just need to know how to format the drive (MBR/GPT?) and how to copy the .iso file to it.
Creating a USB Flash Drive Installer from a Snow Leopard ISO File
This was tested using an iMac (21.5-inch, Mid 2011) 2.5 GHz Intel Core i5 with MacOS High Sierra 10.13.6 installed. The procedure should be the same for Catalina. The flash drive needs to be at least 8 GB in size. The Snow Leopard ISO file was the same as discussed in this answer, which should be the same ISO you have linked to in your question.

Use the Finder application to mount the Snow Leopard ISO file.

Use the Disk Utility application to erase the flash drive, as shown below. I chose the default name Untitled. Finally, click on the Erase button.


Highlight the name Untitled, then click on the Restore button on the top of the Disk Utility application window. In the popup window, select restore from Mac OS X Install DVD, as shown below. Finally, click on the Restore button in the popup window.



Using the Command Line Interface (CLI)

Note: This flashdrive will be using the Apple Partition Map scheme. This differs from the flash drive created using the Disk Utility, which used the Master Boot Record scheme. In other words, there is more than one way to create a USB flash drive installer from a Snow Leopard ISO file.

The steps below are for OS X and macOS.

Use the command given below to determine the identifier for the flash drive.
diskutil list


Enter the commands given below to create the USB flash drive installer. Here, an assumption is made that the ISO file name is snow leopard install.iso and the file resides in your Downloads folder. Also, an assumption is made that the identifier is disk2. If necessary, make the appropriate substitutions.
DISK=disk2
diskutil unmountdisk $DISK
sudo dd if=&quot;$HOME/Downloads/snow leopard install.iso&quot; of=/dev/r$DISK bs=1m



The steps below are for Linux.

Use the command given below to determine the name for the flash drive.
lsblk


Enter the command given below to determine if any volumes on the flash drive are mounted. If there are mounted volumes, then use the sudo unmount command to unmount them before proceeding. Here, an assumption is made that the name for the flash drive is sdb. If necessary, make the appropriate substitutions in the rest of the steps.
mount | grep /dev/sdb


Enter the command given below to create the USB flash drive installer. Here, an assumption is made that the ISO file name is snow leopard install.iso and the file resides in your Downloads folder. If necessary, make the appropriate substitutions.
sudo dd if=&quot;$HOME/Downloads/snow leopard install.iso&quot; of=/dev/sdb bs=1M