Hi! I hope you are doing OK.
If you mean how do your put the R5II into crop mode, In the red section of the menu, On the first page:
Check out the Advance User Guide: P`169: c017.pdf
Note this is simply cropping the image that will be recorded, it's effect is precisely the same as if it was cropped in post processing mode with a resultant reduction in MPs capacity. The main benefit of this is to allow you to crop as you shoot by reducing the Field of View.
I have frequently posted an article on Equivalence: how sensor size changes the field of view and other characteristic. I enclose the link to the article for your convenience:
Equivalence
As regards the following lenses:
EF-S 18-55 is not really a macro lens and as a kit lens designed for APS-C sensors, would be a good candidate to consider replacing with a FF RF lens, such as one of the 24-105 variants. That would work well with either the EF 100-400 or RF 100-500 lenses to give you an unbroken focal range from wide to super telephoto.
The EF100-400II is an excellent lens and will work absolutely fine with the R5II either in crop of FF modes, you just need one of the EF-RF Canon adapters.
The RF 100-500L is also a fabulous optic and of course is specifically designed for the R systems. Apart from the extra 100mm of reach, it also has dual nano-USM motors that make its focusing blazing fast and should be slightly easier to gain and track focus. Really, it depends on what you are going to shoot.
The EF 70-300 is a good lens and should work fine with the R5II - certainly, I would give it a go. Again, it has a nano-USM motor to make its focus fast.
As to exchanging the two, I did post several images in the Gallery > Share Your Photos section showing results from shooting wildlife with the EF100-400 and RF 100-500, and I also did a comparison of the RF 200-800 against the RF 100-500 in 1.6 crop mode.
