Manual CLI configuration of interfaces and features

[dp]

I need to do some testing with few weird configurations that are not possible from the GUI or the console menu. When I research configuration of the interfaces most sites send me to rc.conf to enter the configuration. However when I look at that file it is empty so that means Opnsense is doing it different. Is there any documentation, if it is even possible, on how to do this?

As a side note I do see most of the configuration in the xml file and know most of it is parsed from there and written to the system at boot time. Is there any documentation of what can be added to the xml file?