Since I'm not familiar with networkmanager in combination with other config: what happens if I configure some network interface using Cockpit that is already defined in /etc/network/interfaces.d/*? Will it just throw an error or could it bring my system into a broken/undefined state so that it wouldn't be reachable after the next reboot?
If the latter is the case it would probably be unwise to install/use Cockpit on an already configured system.
Debian appears to require a flag to enable this on its own: https://wiki.debian.org/NetworkManager#Enabling_Interface_Ma...
If it's enabled, networkmanager will parse out the files and present them over dbus, nmcli, etc. Changes with nmcli, gnome-network (which also uses NM), or other tooling will update the config files. The same ones in /etc/network/interfaces.d/
So, hypothetically:
auto eth0
iface eth0 inet static
address 192.168.40.112
netmask 255.255.255.0
gateway 192.168.40.1
dns-nameservers 8.8.8.8
If it's not enabled, Cockpit will not see the interfaces at all.