More details:
If you click on Tags in that dropdown menu, you can then click on 3.4.2-stable and see the source code for that version.
On Linux, you can use the following command to make a local copy of 3.4.2-stable. I find that much more convenient for viewing the source code, since I can easily search for specific functions, etc. The files are fetched into the current directory.
git clone -b 3.4.2-stable https://github.com/godotengine/godot