DMG files are Apple's equivalent of ISO's. Wikipedia explains at https://en.wikipedia.org/wiki/Apple_Disk_Image

If you have a Mac, Disk Utilities will do this for you. For Windows and Linux, you need some additional software. I have not verified the Windows tool, DMG Editor, but an article I read says it is ok. The following is taken from https://www.androidphonesoft.com/resources/burn-dmg-to-usb-drive.html

From Linux, however, you can do the following. You must be the root user.

1. Install dmg2img

   apt install dmg2img

2. Convert dmg file to iso

   dmg2img filename.dmg

3. Determine your usb drive and unmount if necessary

   lsblk

4. Copy the .img file created to your usb drive (assumes sdh from previous step)

   dd if=./filename.img of=/dev/sdh bs=4M

5. Your machine will go into lala land, with no visible indication anything is happening. Just be patient. When done, be sure all data is written with

   sync

6. Remove USB Drive from computer after you get command line back

If the DMG was bootable, you now have a bootable USB.

1. https://www.androidphonesoft.com/resources/burn-dmg-to-usb-drive.html
2. https://en.wikipedia.org/wiki/Apple_Disk_Image
3. https://www.lifewire.com/dmg-file-2620719