In my understanding, an iso file is an image of an optical disc. Namely, a "raw" uncompressed copy of the contents of the disk, which contains a snapshot of some file system such as UDF. Accordingly, I am interested in the following question, is the iso file used only to store the contents of the optical disc? Why then, for example, the ubuntu operating system is stored in iso format? Why not use a different format for storing the zip type? After all, the system often boots via usb (and then the structure of the optical disk is no longer needed)?
It is necessary to dig a little deeper than before the era of laser discs.
Previously, and even now, incl. OS loading occurs as follows (very roughly).
A small code is stored in the ROM BIOS, which, when the computer is turned on, looks for the so-called. MBR – master boot record – a record on the disk where boot information is stored (also essentially a small code). At the same time, what is important, the MBR in the system is located outside of any file system (the file system belongs to the OS). Further from the MBR, you can already run GRUB or the same Windows Boot Manager there and start loading the actual OS itself.
The key topic here is the position of the MBR, in the BIOS settings you can usually specify where the MBR is located: on CDROM / DVD / HDD / SSD / USB, etc. It will no longer be possible to specify ZIP, since ZIP is part of the file system, and the BIOS bootloader does not understand file systems, it can only understand disk sectors.
This is where the importance of the ISO image comes from – the ISO image can save information about disk sectors, respectively, when burned to a disc, it can restore the position of the MBR.
Sooner or later, this will apparently go away, smart BIOSes will appear (and already exist) that can recognize popular file systems, and then already in the BIOS settings it will be possible (in the future) to specify a ZIP archive with MBR, but for now you have to live with what is.