The Startups : Init

Things start up from init.rc. 

If you are interested in learning init language check this link
https://android.googlesource.com/platform/system/core/%2B/master/init/README.md

In stock AOSP this is present in /sytem/core/roodir/

• This imports : import /init.${ro.zygote}.rc 
• Based on you devices ro.zygote system property the init.zygote file from the same folder is imported.

init.${ro.zygote}.rc : looks something like..

service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server

        When asked to start; run zygote program by running "/system/bin/app_process". The remaining part is arguments to app_process

class main

        Adds this to main class; So this service would be started whenever this class is started.

socket zygote stream 660 root system

       Create a zygote socket

onrestart write /sys/android_power/request_state wake

onrestart write /sys/power/state on

      When this service is restarted write these values...

onrestart restart media

onrestart restart netd

      Restart following services too

writepid /dev/cpuset/foreground/tasks

      Write new pid to following path

In the init.rc file you would see the main class being started on the following actions

        on property:vold.decrypt=trigger_restart_min_framework
                class_start main
on nonencrypted
class_start main

etc..
or reset on
   on property:vold.decrypt=trigger_reset_main
          class_reset main
   on property:vold.decrypt=trigger_shutdown_framework
          class_reset main
etc..

The zygote service can also be restarted if they are marked for restart by some other srvices like...
service servicemanager /system/bin/servicemanager
service surfaceflinger /system/bin/surfaceflinger
etc..(So if service manager or surface flinger dies and rrestars the zygote would also restart)

P.S Things might vary based on the init.rc files on different devices.