The amalgamation file is a really interesting idea -- is this common in the world of C applications? The documentation is quite clear, however, that the amalgamation file and the source files are not the same thing. The source code (1,848 files in 40 folders) can be pulled down here -- https://www.sqlite.org/cgi/src/doc/trunk/README.md -- but more assembly will be required if you're planning to build the project.
UPDATE: maybe not so much assembly is required... just running "make" built the project without any drama (I'm on macOS with XCode and tooling for Xamarin already installed - YMMV in terms whether you might need to install something to compile from source).
It's not common, but there are other projects like this for easy integration, especially on embedded systems. You see some C++ libraries that are a single header file include. And here is a collection of single file headers for graphics tasks, like loading images, resizing images, rendering font glyphs, etc: