When you want to create a persistent live USB drive you can use
The Ubuntu Startup Disk Creator and Disks (alias gnome-disks) are cloning tools, and cloning does not create persistent live drives from the Ubuntu [and Ubuntu family flavours] iso files.
Cloning works with all hybrid iso files in order to create USB boot drives, and most Linux iso files are hybrid files. There is a test in the Startup Disk Creator to check that it is an Ubuntu [or Ubuntu family flavour] iso file. If that test would be removed, it would work for all hybrid iso files.
Both mkusb and Rufus can clone (dd-mode in Rufus), but they can also use methods that modify what is written to the USB pendrive.
You can find detailed information and several links at