Cisco gets super weird because they didn't want to keep supporting new hardware, so back in the mid-2000s they actually started running a Linux kernel. There was some guy at Kiwicon 2016 (I think) who showed how he hacked a Cisco router and broke out of ios into the base Linux shell.

The base Linux only shows once Ethernet adapter because the others are implemented using PCI-E userspace drivers (which is apparently a thing) and ios is one massive binary (I think it was a few hundred megs) that's run as the only process.

They somehow adapted the entire OS to run as a userspace process on a Linux kernel.