Hallo Kai!

 KR> a basic shell system of 300-400MB where you can install your
 KR> server on top.

I seriously doubt the above contains gcc and friends which as of gcc-9.2.0
takes up 455MB for the basic toolchain which in my humble guesstimation is the
bare bones minimum to call a linux-gnu shell enviroment.  Without gcc and
friends the above is only a toy and doesn't even rate calling a evaluation
system given the lack of development tools.  Anything that cannot take care of
itself isn't worth booting in the first place.

 KR> From scratch is a far longer journey but you'll learn many
 KR> insides of the system.

Which is what was asked for in the initial enquiry.  I'd argue that it is a
worthwhile document just for reading but if and when followed will provide one
with the best possible system with whatever additional sources one adds to it.

Het leven is goed,
Maurice

... Huil niet om mij, ik heb vi.
--- GNU bash, version 5.0.11(1)-release (x86_64-pc-linux-gnu)