Add more information on building.

Author: jonsimantov

AGENTS.md

@@ -42,6 +42,12 @@ instructions for your specific platform.
     Android builds on Windows.
 *   **Cocoapods**: Required for building iOS or tvOS libraries.
 
+To build for Desktop, you can install prerequisites by running the following
+script in the root of the repository: `scripts/gha/install_prereqs_desktop.py`
+
+To build for Android, you can install prerequisites by running the following
+script in the root of the repository: `build_scripts/android/install_prereqs.sh`
+
 ## Building the SDK
 
 The SDK uses CMake for C++ compilation and Gradle for Android-specific parts.
@@ -50,15 +56,16 @@ The SDK uses CMake for C++ compilation and Gradle for Android-specific parts.
 
 1.  Create a build directory (e.g., `mkdir desktop_build && cd desktop_build`).
 2.  Run CMake to configure: `cmake ..`
-    *   For iOS:
-        `cmake -DCMAKE_TOOLCHAIN_FILE=../cmake/toolchains/ios.cmake ..`
-        Note: iOS setup typically requires both including Firebase pods (via
-        `Podfile`) and linking the `.framework` files from the C++ SDK
-        distribution.
-    *   For tvOS:
-        `cmake -DCMAKE_TOOLCHAIN_FILE=../cmake/toolchains/apple.toolchain.cmake -DPLATFORM=TVOS ..`
+    *   For Desktop: Run as is. You can use BORINGSSL instead of OpenSSL (for fewer
+        system dependencies with the `-DFIREBASE_USE_BORINGSSL=YES` parameter.
+    *   For iOS, include the `-DCMAKE_TOOLCHAIN_FILE=../cmake/toolchains/ios.cmake`
+        parameter. This requires running on a Mac build machine.
 3.  Build specific targets: `cmake --build . --target firebase_analytics`
     (replace `firebase_analytics` with the desired library).
+    Or omit the entire `--target` parameter to build all targets.
+
+You can also use the `scripts/gha/build_desktop.py` script to build the full
+desktop SDK.
 
 Refer to `README.md` for details on CMake generators and providing custom
 third-party dependency locations.
@@ -76,6 +83,9 @@ This command should be run from the root of the repository. Proguard files are
 generated in each library's build directory (e.g.,
 `analytics/build/analytics.pro`).
 
+You can build the entire SDK for Android by running `./gradlew build` or
+`build_scripts/android/build.sh`.
+
 ### Desktop Platform Setup Details
 
 When setting up for desktop, if you are using an iOS