How to write custom software for my smartphone?

Question

Android

Lord, tell me please. I have a Chinese smartphone on the MTK6572 processor, as I understand it, this is an SoC, i.e. the chip, among other things, includes both GSM / HSPA, and a video controller, etc. I want to do the following as an experiment: write something in ARM assembler or in C, such as a bootloader that displays something on the screen. How can we even approach this problem? Well, i.e. how to write instructions in assembler is understandable, but how to understand how to send commands to the video controller, etc.? Where to dig at all (what to write in Google)? I found a schematic diagram on the network of this processor and some sorts, but it is not very clear how to use all this. Help me please. Intuitively, it seems that you need to look for something like "how to make my own rom", but all the links in Google on this topic lead to how to flash an already finished rom, finishing it a little. In addition, as I understand it, for this it is not at all necessary to make a ROM, there is no need to specifically sew a processor, but you can simply write code that will run from the NAND partition, right? It's still not clear, MTK6572 and MT6572 are the same thing? And then there are two names.
Thanks in advance and sorry for wasting your time!

Answer 1

What MT? What firmware, what NAND partitions? What are you speaking about?
1. Take Android NDK.
2. Write something in C, like printf("Helloworld").
You can also write in ARM assembler, and x86 too - everything that is in the GNU Toolchain is there.
3. Edit Android.mk to compile to Executable instead of Shared object, that is, instead of a library (Android NDK is "sharpened" for libraries, if you compile to a library, you will also have to write a "wrapper" in Java using JNI)
4. Then open adb shell( it is desirable to have root rights - it is inconvenient without them) and run the resulting executable.
Everything! The screen will say "Hello World", and I assure you, this will happen with all the necessary commands sent to the processor! Moreover, even if you write in Java, it still won't work without a processor! And even if you make a page in HTML & CSS & JS and open it in an Android browser, everything will still work through the processor!
All the task you set is completed, if you want this to happen instead of starting the OS - so "solder" this matter into the OS and that's it, it has an open source code.
Android is Linux, everything described works on any Linux, and on it too.

Answer 2

After uploading an untested firmware directly into a live phone, it will turn into a brick with a 99.99% probability.
So no one develops firmware, including those who develop them professionally.
Where to start? - From the purchase of a special Development Board for the target SoC, for example .