Towards a custom personalized Debian installer

[Richard Owlett <rowlett@cloud85.net>]

The default of copying an ISO file to a device is inconvenient for my 
peculiar goals.
I want an executable installer resident on an ext4 formatted partition.
It must be possible. A testable installer preceded ISO format.
I'm working thru https://www.debian.org/releases/stable/amd64/ch04s03,
in particular extracting steps for "Manually copying files to the USB 
stick — the flexible way".

What reference am I missing?

TIA