I ran across a video that shows a basic Wi-Fi router connected as a access point from the Internet port to a switch From a PFsense router. How is this even possible? Every single recommendation I have seen for an access point always recommend using the LAN port and not the Internet port. Looking for instructions on using my old wifi router for ap but this confused me. Any ideas?

https://youtu.be/iS9Sb7qjRbo?si=agLLcOCgmQQT6pB2