I wrote a web ui to manage my home server, this project was just for fun. I'm probably not going to end up actually using it.
But I would like to learn how I could make my own custom debian installer whcih has the web ui installed.
I figured I need to use live-build for this:
lb config --binary-images iso-hybrid --mode debian --architectures amd64 --linux-flavours amd64 --distribution bullseye --archive-areas "main contrib non-free" --updates true --security true --cache true --apt-recommends true --debian-installer live --debian-installer-gui true --win32-loader false

I included the packages that I want and the package with my custom webui.
This all works but there are a couple of things I want to change.
- The debian-installer still has the debian logo, I would like to replace this.
- The grub installation (done by debian-installer) shows the boot options with the name "Debian GNU/Linux". I found out that grub-install has an option bootloader-id to change this. But I have no idea where I could change this inside the debian-installer.

I think I'll need to get the source code of debian-installer and edit it to fit my needs? But I couldn't find 'grub-install' inside that source code

edit the grub.cfg file on the iso, I think it will be in boot/grub for uefi boot, for legacy boot it will be a config file in isolinux directory.

That's something I was also thinking of. But the grub.cfg file in the /boot/grub directory has the options to load the installer.
- Live system
- Graphical installer
And those options aren't on the installed system.

All this does is change the name of the EFI/debian directory to something else. Grub menuentry options along with the debian logo displayed are controlled by boot/grub/grub.cfg and/or a grub.cfg somewhere in a sub-directory of the efi directory or another .cfg file sourced by the grub.cfg

don't know about the debian installer grubx64.efi, some will have a grub.cfg embedded in the grubx64.efi

I think I'll just end up writing my own installation script.
When I boot the live-system I can install it onto the hdd using debootstrap.
This would give me more freedom to customize whatever I want.
Once I've made a script for it. Can I make run it automatically when booting the live-system?

Look at config files in /usr/share/live/build/bootloaders/grub-pc, the /usr/lib/live/build/binary_grub_cfg file and the livebuild common config file.