"Many HDMI 2.1 features will rely on ultra-high speed bandwidth, so don’t forget about upgrading your in-home networking with better cable modems and service, Wi-Fi routers and multi-room connectivity. "
This is a false statement. You don't need to change ANYTHING in your home networking, cable modem, or WiFi. HDMI 2.1 is just for the cable to connect from your input device (PS5, XSX, etc.) to your display device (TV, etc.). You don't need to worry about anything in the above quoted sentence. It's almost like some person inserted this statement into the middle of a marketing blurb, just so they could encourage buying other "stuff".
Now, one thing that isn't listed, is that you may need to confirm if your AV receiver supports HDMI 2.1, if you want to connect to/through it. There have been some chip problems with some of the earlier models claiming HDMI 2.1 support, so do your research first. Most of the new(er) 2021 models shouldn't have the issue, if they support HDMI 2.1 connections at all, but just investigate & confirm your desired model does.