First download UBCD (if you didn't do it yet).
Look at the files and programs you see.
Search info about the programs on the internet.
Try by looking at the "source" code, to see how it works.
The image itself: /ubcd/images/dosubcd.igz
The dos applications that use it: /ubcd/dosapps/
Used as generic bootimage for a lot of dos applications.
The menu is made with syslinux:
/isolinux and /syslinux contain the programs of syslinux, /ubcd/menus/ contains the menu entries.
The menu is one of the most important parts of UBCD. Look and study how you have to use the parameters. It also gives information about which files are needed to get some program to work.
/ubcd/images/ contain gzipped floppy images. To decompress them, rename .igz to .img.gz
See also the Useful link page ==> Working with floppy images
on the homepage.
/ubcd/boot/ contains various linux programs, which consist of a kernel and an initrd.