I have an M1 MBP. I connect it to my external display (DELL S2721QS) using a HyperDrive Viper hub.
When my MBP is in clamshell mode, the external display works at 4K 60Hz resolution. When the MBP is open, the external display only works at 4K 30Hz.
My hub and HDMI cable are both capable of 60Hz, as shown by the fact that it works when in clamshell mode. Does anyone know if the M1 MBP is capable of driving a 4K display at 60Hz when the laptop is open?