@777: 120hz refresh rate should already be accessible via Settings > Display > Motion Smoothness, however this will reduce the overall resolution to 1920 X 1080. Apologies for any disappointment this may cause.
1210hz qhd cannot be achieved by default and tricks which enable it reduce image accuracy since there is not enough badwidth for both with the screen chip in the s20, however i would like to underline that detecting a difference between 1080p and 1440p on such small screens like phone are hardly to no detactable for most people even at unreasonably close distances, try it yourself and probably you will not notice difference between 1440p and 1080, so if do not notice difference do not worry too much about 1400p at 120hz on smartphone, it is mostly gimmick.
Have a nice day!
never looked for a guide about that because i have a galaxy s10 however i would look on xda.
When the s20 came out they made a script to enable 90hz on the s20 lineup and i think that the 120hz wqhd trick is still from them