Here you can download binary distributions of the game for Linux or Windows compiled for x86_64, or download the source code and compile it yourself.
The game is released under the GNU AGPLv3.
The recommended way to install Spaceships on Linux is through Flatpak as it provides sandboxing for greater security and updating it is much easier.
If you haven't got Flatpak installed, go here to get it. After that, download this file and open it to install it through your GUI package manager. Alternatively if that doesn't work or you don't have a GUI package manager, run flatpak install --user spaceships.flatpakref with this downloaded file.
Another alternative is to run these two commands which installs the repository and the application:
flatpak remote-add --if-not-exists --user spaceships https://spaceships.me/flatpak/repo.flatpakrepo flatpak install --user spaceships me.spaceships.spaceships
After installing it, the game is available in the menu that your desktop environment shows applications. If not using a desktop environment, you can run flatpak run me.spaceships.spaceships.
Alternatively get the repository at https://codeberg.org/rustydev/spaceships.
Here you can download offline copies of the documentation. You can get the documentation for the latest version online here.
I haven't yet tried compiling this game for macOS, so I don't know if it will work. If you're using macOS, feel free to compile it yourself and see if it works.