bwebel

Shimano 12 speed road cassettes fit the same freehub as their 11 speed cassettes. Most SRAM do not, though they just came out will a couple of ones that do. SRAM insisted on 10t cogs for their 12 speed, which you can't put on the freehub body size that have been in use for the last 30-odd years. (they also insisted on an incompatible chain; kind of ironic given SRAM's history of copying Shimano, who periodically tried to create incompatibilities so SRAM couldn't do so)

I've never use SRAM 12 speed shifting with a Shimano 12 cassette, but most things I've seen indicate that they are pretty much interchangeable. SRAM's shifting worked pretty well on the Rotor 12 speed cassette when I used it.