If you want to learn the system from scratch, the best way will be writing your own little init system from scratch, so you can understand how the boot sequence works. And as you make use of more and more of the advanced features of Linux, your init system will get more and more complex, and will start to resemble systemd.

If you only learn about sysvinit and stop there, you are missing large parts of how a modern Linux distro boots and manages services.

> and will start to resemble systemd

That's the point on which people differ. Even if we take as given that rc/svinit/runit/etc is not good enough (and I don't think that's been established), there are lots of directions you can go from there, with systemd just one of them.