The Samsung Galaxy S20-series launched with almost flawless displays, but they had a somewhat strange limitation. To run the devices at 120Hz, the Galaxy S20 trio has to tone down to 1080p resolution. With the Galaxy S21 family, or at least the S21 Ultra, you will be able to keep the resolution at WQHD+ while benefiting from that smooth, adaptive refresh rate at 120Hz.
A screenshot from the settings menu posted by Ice Universe confirms those claims showing that the adaptive refresh rate is on while the resolution is 1440 x 3200px.
The S21 and S21+ are rumored to get 1080p panels in the first place, so the limitation wouldn’t have applied to them anyway.