As well as for the engine itself, you will need to configure the environment. And also you may need to apply several patches:

cd godot-cpp
# Optional
# #Build only the necessary classes
git apply --ignore-space-change --ignore-whitespace ../patches/godot_cpp_exclude_unused_classes.patch
# #Faster builds and larger binaries
git apply --ignore-space-change --ignore-whitespace ../patches/unity_build.patch
# #Hide useless function exports. Can greatly reduce the size of libraries
git apply --ignore-space-change --ignore-whitespace ../patches/visibilty_hidden.patch

Then you can just run scons as usual:

# build for the current system.
# target=editor is used for both the editor and the debug template.
scons target=editor dev_build=yes debug_symbols=yes
# Android build. ANDROID_NDK_ROOT is required in your environment variables.
# Or ANDROID_HOME with 'ndk/23.2.8568313' installed.
scons platform=android target=template_release arch=arm64v8
# Web build. GDExtension for Web currently works best with Emscripten '3.1.39'
scons platform=web target=template_debug

JavaScript/Web build

If you have problems running the Web version of your project, you can try using the scripts and tips from this page.

If you too want to add an Interactive Demo to your GitHub repository, then you can see how Nick Maltbie (nicholas-maltbie) added this feature to this repository in PR #24!

In short, you need to activate Extension Support when exporting and add the gzuidhof/coi-serviceworker to the <head> and to the export folder. Then you will need to somehow publish a demo on the GitHub pages, as for example done in #24.