We generally follow the contribution guidelines of rust-bitcoin.
We use just for running development workflow commands. Run just from your shell to see the list of available commands.
The bitcoind-tests/ package contains integration tests that run against real Bitcoin Core instances. A separate package is used so that bitcoind version flags don't pollute the rust-psbt crate. The package is not a member of the workspace so that it doesn't effect dependency version resolution.
The auto-downloaded Bitcoin Core binaries don't work on NixOS due to dynamic linking requirements. If you're on NixOS you could manually configure the BITCOIND_EXE environment variable to use a Nix-provided bitcoind of the correct version.