Configuration folders

Is there documentation that somehow describes what should go in any of these?

  1. /etc/sysconfig/
  2. /etc/default
  3. /etc/systemd/system.control
  4. and /etc/systemd/system or /etc/systemd/user

That depends on the package that installed the configuration files. With some files, you can display it and see what it contains.

For /etc/default/grub you can see GNU GRUB Manual 2.14 or run info grub2.

Systemd has extensive documentation. See https://systemd.io/

I think this is considered legacy these days, atleast for some config files.
I am seeing some file there are symlinks to other folders in /etc.

you could see what package creates the folders/files, .e.g.

rpm -qf /etc/default/

=> filesystem-3.18-52.fc44.x86_64

on Fedora 44

Thanks!. I was looking at RH/Fedoras docs and couldn´t find anything definitive.