You can play at 120Hz, but this does not affect the performance of your PS. In addition, the "120Hz mode" is only activated automatically for certain games, you cannot select it manually.

e.g. COD Cold War runs on this screen at 120hz, but you have to switch on the power mode in the settings of the PS5. It even works if you have to split the HDMI signal of the PS5 with an adapter due to the missing optical output (e.g. if you use an Astro headset).