Side question - I have always wondered how a Linux system is configured at the lowest level?
Let's take example of network. There's IP address, gateway, DNS, routes etc. Depending on distribution we might see something like netplan reading config files and then calling ABI functions?
Or Linux kernel directly also reads some config files? Probably not...
Linux kernel as much as possible tries not to parse or read external data (besides stuff like acpi tables, device trees, hardware registers). For networking, you might look at the iproute codebase to see how they do things like bring a network device up, or create a bridge device, add a route, et cetera.
Edit: looks like iproute2 uses NETLINK, but non-networking tools might use syscalls or device ioctls.