Fake ethernet interfaces (feth / if_fake)? Anyone ever seen this?
Yes you can create new interfaces and link them however you would like for whatever your heart desires : ) You can utilize this feature for all different things like isolating certain traffic. For ex:  forcing all traffic through a VPN except for some criteria. My router sends all traffic through a VPN which is a separate LAN from my STB/Cable Box. I want to use my mobile device to change the channel but it won’t work if I’m in a VPN, a separate interface can ether tunnel traffic from my device or isolate traffic directed to the mobile app I’m using and route it around the VPN. 

As for reference and documentation, Unix utilizes man pages like Linux too for basic and core reference. I like to use cheat.sh for examples, it’s formatted very nicely too when you make a query within your terminal. It’s formatted like this cheat.sh/example. ~ cheat.sh/ifconfig