The standard on the port and it’s supported resolutions / refresh rate will matter. Plugging in the monitor and running xrandr will show the supported resolutions and refresh rates for the monitor, as this is exposed to the computer via EDID.
Depending on what resolution you are looking at you should be able to do 120Hz over DP. A 4k monitor or smaller should be possible, but larger is iffy: External monitor support on the Framework Laptop
HDMI may be limited to 60Hz at 4k with a HDMI expansion card and I’m unsure if 120Hz is supported below that. A USB C dock may be capable of 120Hz. Take this with a grain of salt.
This post shows maximum display port output speed for a few monitor sizes from AMD for the 7840hs (essentially the same as the U):
In theory could do 4x 4k-120Hz monitors simultaneously (including the internal panel) over DP.