How to write an iso image to a usb flash drive and add files to the image?

Anybody know how to add files to a bootable iso image before or after it's flashed to the usb drive? In Windows this is easy to do, but I've had no luck yet in Linux (Zorin). I used BalenaEtcher to flash the drive. This program formats the drive so it's exactly the same size as the iso image. It also makes the flash drive read only so I can't do any changes to the usb drive. Help will be much appreciated.

This was the first thing on a simple google search, I'm sure there are others

https://www.techwalla.com/articles/how-to-add-files-to-an-iso-image-in-linux

Depends on what distribution you want? Example propably archinstall is for arch.
Fedora is "everything"
Ubuntu is Create Your Own Distribution Based on Ubuntu
Propably somewhere here i put some tutorial.
Remastered Zorin

I get an error message when I try to mount the iso image: "failed to setup loop device for ...". I've triple checked that the location and the path is correct. I will investigate further to see if I can find a solution. Btw, the iso image is a Windows 11 installation. Thank you for your reply.