That's not the best video. It keeps mixing a mini-pc with 2 NICs with one with 4 NICs. I think he used a 4-NIC mini-pc in the end. You can add another NIC by attaching a RJ45 USB adapter and using it for OPT1. Follow these instructions instead. https://www.zenarmor.com/docs/network-security-tutorials/how-to-configure-transparent-filtering-bridge-on-opnsense