Thank you for purchasing.
It's the Godot project file itself. You can just download the project folder and unzip to your current project (or inside a new project - recommended), then you can use the assets as it is. However the demo (it's a standalone playable exe) is for Windows computers only.
This should technically work well with 4.5 (I do use 4.5 myself and no bugs yet). But with the 4.6 update, I'm planning to restructure and optimize the assets (more polish/ etc.)