added some basic dir infos in source

aosp-source-tree.md

@@ -210,10 +210,11 @@ Depending on the branch or target (like Pixel or emulator), you might also see:
 
 - **Purpose:** Hosts the source code for system apps and UI components.
 
+
 - **Key Examples:**
   - `Settings/`, `SystemUI/`, `Launcher3/`
 
-- **Why it matters:** This is where things like the lock screen, Quick Settings, and launcher behavior live.
+- **Why it matters:** This is where things like the lock screen, Quick Settings, and launcher behavior live. You can add most required apps that you want to be preloaded.
 
 
 
@@ -228,10 +229,83 @@ Depending on the branch or target (like Pixel or emulator), you might also see:
 ### 16. `prebuilts/` – Precompiled Tools
 
 - **Purpose:** Tools and binaries that don’t need to be built from source (e.g., clang, aapt).
+You can also make or modify and use your it as you wish but its kinda hardcoding skills needed i recommend to stick with aosp ones as they are stable.
 
 - **Structure:**
   - `clang/`, `gcc/`, `sdk/`, `tools/`
 
-- **Why it matters:** These speed up builds and ensure consistent toolchains.
+- **Why it matters:** These speed up builds and ensure consistent toolchains. 
 
 
+
+### 17. `sdk/` – Android SDK
+- **Purpose:** Contains parts of the Software Development Kit used by app developers.
+
+- **Key Components:**
+  - APIs, emulator images, tools like `aapt`, `dx`, and more.
+
+- **Why it matters:** This is how Android app devs build, package, and test their apps using the platform you’re modifying.
+
+
+
+### 18. `system/` – Core Android System
+
+- **Purpose:** One of the *most important* folders — includes Android’s `init`, core utilities, security policies, and daemon logic.
+
+- **Key Components:**
+  - `core/`: Native system components.
+  - `sepolicy/`: SELinux rules for enforcing mandatory access control.
+  - `init/`: The native `init` process implementation.
+
+- **Why it matters:** Everything from system service spawning to filesystem mounting begins here.
+
+
+
+### 19. `test/` – Unit & Integration Testing
+
+- **Purpose:** Used for testing Android components at the system level.
+
+- **Includes:**
+  - JUnit-based unit tests, integration frameworks, shell test runners.
+
+- **Why it matters:** Ensures reliability when changes are made to core components.
+
+
+
+### 20. `toolchain/` – Compiler Infrastructure
+
+- **Purpose:** Holds source and configs for GCC/Clang-based toolchains.
+
+- **Why it matters:** You might not need to edit this often, but it defines the compilers used when building native code.
+
+
+
+### 21. `tools/` – Build Tools and Utilities
+
+- **Purpose:** Miscellaneous tools used for code analysis, linting, SDK packaging, etc.
+
+- **Notable Subdirs:**
+  - `repohooks/`, `lint/`, `apkzlib/`
+
+- **Why it matters:** Improves code quality and build consistency across the platform.
+
+
+
+### 22. `vendor/` – Proprietary Vendor Code
+
+- **Purpose:** Non-open-source components and binaries provided by chipset vendors or OEMs.
+
+- **Why it matters:** The bridge between AOSP and real hardware. If you want to port Android to a new device, you’ll need matching vendor blobs here.
+
+
+
+### (Optional) `product/`, `vndk/`, `common/`, `soong/`
+- **Seen in some branches or manifests.**
+- `product/`: Used for `/product` partition overlays.
+- `vndk/`: Vendor Native Development Kit under Project Treble.
+- `common/`: Shared code across multiple device platforms.
+- `soong/`: Standalone directory for the Soong build system, in some branches.
+
+
+
+Mostly this is much you learnt is enough for now but you might need to learn the much deep inside each folders when it comes to customizing, but no worries i will take care of it. I will also tell you how to customize of things and behaviours later on upcoming chapters. Stay Tuned!