APP - Root/unroot any device from the app, no PC required

I am a newbie with Android programming, quite franckly, I dont even know half the basics, but so far I have create an app and is in the making, it can either apply a temporary root or a permanent root with a custom recovery of choice aswel

However, I would like to find out, what is rooting acutally, what exactly does it unlock or does it just change the /sys partition to +755

I know with Samsung, all you need is flash a custom recovery .tar.md5 and apply a SPECIFIC su zip file, any other file wont work,

with HTC I had to unlock the bootloader, then flash a custom recovery and so on

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TL;DR Now my question: What is done to a device when it is rooted, not just it e.g. grants access to /sys, but a more indepth info in the changes that are applied

or atleast a refernece to where I can do more research

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p.s. I have the superuser source code, but I don't want to waste my time looking for something someone else already knows