/src/ contains most of the source code for the project.
/src/main.rs is technically the entry point of the project (when running it directly), but the real "root" of the project is /src/lib.rs.
The comments in that file should guide you around (hopefully).
/tests/ contains integration tests.
Make sure you have a STABLE (not NIGHTLY) rust toolchain installed and set up properly, as well as cargo installed.
Running cargo run will run main.rs, which may or may not be interesting.
Make sure to go and uncomment some stuff in there for more interesting results.
Running cargo test will run all the tests.
Make sure flakes are enabled in your config. The flake.nix here provides a default package and a dev environment.
Running nix develop will put you in a shell with the proper cargo set up, and you can run the commands mentioned above.
Running nix build / nix run will build/run the project.