If you want to build your own ISO then you need to look into live-build. Debian have some excellent documentation on this. Ubuntu's live-build package does things slightly differently, but most of debian's documentation still applies. The scripts Ubuntu uses is in the livecd-rootfs package. You copy the auto folder from this package to your home folder and then run the live build commands on it.
Have a look at the scripts in Status of Ubuntu MATE 18.04 for Raspberry Pi 3 B & B+? - #8 by stillwinter . They are for the raspberry pi so don't actually produce an ISO, but if you take the build-ISO script, change the architecture to i386 and remove other pi bits then after you run it then you should get an ISO file. This is essentially how official Ubuntu ISOs are produced, except Ubuntu's automated build scripts do other stuff such as adding efi and changing the hash files etc (the complete-ISO file does some of this, except again it doesn't produce an actual ISO....).
Kde neon have documented some of this too.