Android device load process (preparing flashing)

Boot process of Android devices up to kernel stage is very vendor specific, device specific and contains mostly closed source code. You may see clear deviations of terminologies used and differences in their implementations. Here I'm giving a simple understanding.

1. When charger is inserted, device boots very similar to when power button is pressed except that a very limited OS is loaded.

   
   
   

   Device boots normally up to init stage (given at point 4). init, instead of starting services from /system and /vendor partitions, starts a process: /sbin/charger* which is not a part of AOSP. Charger binary is usually closed source and varies for different devices. It may load some pictures / animations stored on some partitions specific for this purpose.

   
   
   

   Also some OEMs allow enabling / disabling off-mode-charge through fastboot to control the behavior i.e. whether the device should be switched on when power is plugged in or not. But it's also not a standard feature and varies for devices.



2. Bootloaders (SBL / ABOOT or equivalents) handle Power and Volume buttons pressed to start (Emergency) Download mode, SPL mode, fastboot mode etc. and recovery or normal kernel.


3. Preloader is a bootloader stage (SBL / SPL) before final bootloader (UBOOT / LK / ABOOT)


4. Here is the boot chain:
   
   
   
   
   • BootROM
   
   
   • Primary / Secondary BooLoader or PreLoader
   
   
   • Final bootloader (which provides odin / fastboot)
   
   
   • Kernel / Recovery (TWRP)
   
   
   • Init (first userspace process and in-charge of all other processes, also mounts filesystems)
   
   
   • Services or processes are started from /system and /vendor partitions by init (Zygote / Dalvik is one of those).
   
   

So odin (specific to Samsung) and fastboot are two independent protocols for communication with device through USB to format partitions, flash factory firmware, (un)lock bootloader etc. While TWRP is a minimal OS which is booted in place of normal boot.img (usually using same kernel) in case Android is unable to boot or needs OTA update or some modifications / maintenance.

None of them can be related to Dalvik, they are hardware specific and run at pre-kernel or kernel stage. While zygote - a part of AOSP - is a completely userspace process which forks Dalvik Virtual Machine for every app.

For details: Boot Process: Android vs. Linux