Forgive my ignorance, this isn't my strong suit. Am I correct in understanding that this is mostly a simulation layer for the actual physical network, but that you're mostly(?) running off-the-shelf software on top? So this is running the same software that you'd use for a real ISP network, just without having to actually provision all the hardare? Or is part of the actual network management custom as well?

Hello. Containerlab gives me the virtual network topology ( links through veth pairs, containers etc.). The actual BNG's Control plane ( authentication, authorization, session handling, traffic shaping, events streaming etc. ) is written by me. So it's less running off-the-shelf software running on virtualized hardware, and more writing the software and running it on a virtualized hardware.

At some point, I did use Nokia SR Linux as my access node + relay, but had issues with configuration and Option 82. Later, I wrote one myself.