Edir: i see this was already mentioned.

Not sure if you meant the video, or written guide, but for the written guide -

OPNsense is not even mentioned.

When we build a router using a standard computer, we can install router software like pfSense or OPNsense,

There’s a bit of a debate between pfSense and OPNsense. TL;DR, the developers of pfSense are not the nicest people sometimes. If this bothers you, consider checking out OPNsense. Since I’ve been using pfSense for a decade, I’ve built much of my infrastructure around it. I am well aware of its quirks and don’t feel like setting up my network from scratch, so I am using pfSense for this tutorial. Regardless of the developers, you are infinitely better off using pfSense on your own hardware than standard routers.