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

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.