What happened when the user space application accesss device registers via mapped memory on /dev/mem?

Hi all, I want to know the details of device(not physical ram device) registers via mapped memory on /dev/mem. For example, a Soc CPU has some internal device register resources, the driver will map the memory block to a virtual memory to kernel. But, the user application can use /dev/mem to access it(devmem2). I want to know why? and what's happened on the backend? Thanks.