Below is a visual example of how a typicals smartphone internal storage can look like. Since every device may look different this is merely to give you an overview about how a typical Android phone is structured.
The graphic below is a simplified overview of the steps necessary tu install a custom operating system onto a phone. Unfortunately some vendors decide to make it extra hard for their customers to do so, so a simple step like unlocking the bootloader might end up taking multiple steps.
Next Page: What “adb” is and how to prepare it