The boot sequence of the Raspberry Pi is basically this:

1. Stage 1 boot is in the on-chip ROM. Loads Stage 2 in the L2 cache
2. Stage 2 is bootcode.bin. Enables SDRAM and loads Stage 3
3. Stage 3 is loader.bin. It knows about the .elf format and loads start.elf
4. start.elf loads kernel.img. It then also reads config.txt, cmdline.txt and bcm2835.dtb If the dtb file exists, it is loaded at 0×100 & kernel @ 0×8000 If disable_commandline_tags is set it loads kernel @ 0×0 Otherwise it loads kernel @ 0×8000 and put ATAGS at 0×100
5. kernel.img is then run on the ARM.

Everything is run on the GPU until kernel.img is loaded on the ARM.

there are  6 Stages of Linux Boot Process, which also contains a full explaination of each stage. 

sudo stat /proc/1/exe

apt-cache show systemd

Now that we see how to get up to the Systemd initialization lets have a look at some commands we can execute on systemd to see how it is working.

Awesome ! systemd Commands to Manage Linux System list the commands I am going to try on a raspberry pi.

View a tree of running processes: pstree.

How long to start up: systemd-analyze.

Which services took how long to start: systemd-analyze blame.

What units are running: systemctl list-units.

Which services are running: systemctl list-units -t service.

What is the status of a service: systemctl status udev.

Finally let me give you a couple more links of summary information about systemd.

• systemd Cheat Sheet for Red Hat Enterprise Linux 7

• Useful SystemD commands (hints for systemctl or systemctl vs chkconfig and service)

change default boot account

Edit /etc/lightdm/lightdm.conf

Set autologin-user to the new user.

Restart X or reboot