In the book Embedded Android, it says
In init.rc, only an action results in the execution of commands. Service declarations only serve to describe services, they do not actually start anything... (page 246).
So, where are the Android system services (e.g., servicemanager, vold) started? I thought they were started in init.rc by the following lines. But according to the book, I was wrong.
service servicemanager \/system/bin/servicemanager
service vold /system/bin/vold
I found the answer.
The statement "Service declarations only serve to describe services, they do not actually start anything" is correct. The native daemons are actually started in init.rc by following lines: (lines 371-372 in https://android.googlesource.com/platform/system/core/+/master/rootdir/init.rc)
where all daemons that belong to class core and class main are started respectively.