First of all, the Mark II sticks with 45 megapixels. I am pleased by this -- it indicates that Canon isn't simply trying to use pixel count to sell new models. Early speculation was that the Mark II would jump to 61 megapixels.
The mark II has what they call a "stacked sensor" which somehow allows faster readout speeds. This can give you 30 fps shooting (versus 20 fps with the original) with the electronic shutter. It also allows 8K 60p video. Faster readout speeds mitigate the problem with rolling shutter distortion.
Note that when the R5 gives you 20 fps with the electronic shutter, you get it in 12 bit mode. The R5ii gives you 14 bit with 30 fps via the electronic shutter. With mechanical shutter, either camera gives you 6.3 fps and 14 bit.
To go with this, they also give you an improved AI driven autofocus called "action priority".
All of this is good for the action shooters. Perhaps birds in flight. Certainly sports shooters.
The body is redesigned and sleeker, more like the R3. (The R3 is a 24 mpx full frame camera introduced in 2021). They moved the power switch to the right side, replacing it with a movie/still switch. There are plenty of other minor tweaks and features, none world changing.
Tom's Digital Photography Info / tom@mmto.org