For <( ... ):
  
  1. parent: pipe2([3, 4])
  2. parent: dup2(3, 63)
  3. parent: close(3)
  4. parent: fork()
  4. child:  dup2(4, 1) # now STDOUT is the writing end
  5. child:  proceed to ...
  6. parent: close(4)
  7. parent: openat("/dev/fd/63", O_RDONLY) and expand as "/dev/fd/63"

For >(...):

Much the same but read/write fd's are inverted and #7 opens with O_WRONLY.