This TV *cannot* do 120fps with Xbox Series X or PS5. Although this set DOES have HDMI 2.1, and yes HDMI 2.1 can support 120hz (aka 120fps), isn't supported on ALL sets that have 2.1 because there is no standard requiring it. It's ridiculous really, and is only going to get worse. Now coming up there is HDMI 2.1a that will be out soon and technically all ports moving forward will be called that and they don't have to support 120hz either because there are no requirements set for the HDMI standards. It's a mess and TV manufacturers aren't clear enough about what their sets can and cannot do. It's a huge pet peeve of mine on the HDMI standard because it's so ridiculous. One day this has just got to change, but sadly, that day is not today.
Just to add that although the HDMI ports wont support 120hz, this TV set will support it if the game is downloaded from amazon directly to the tv via it's app. The game would have to support 120fps, of course. But the rule of thumb with this set is if it's plugged in via HDMI, it's 60hz.