Hi! Project Isolation is a fully open-source FPS. You can use it to study, learn, or even build your own games off of! However, if you'd like to create your own games based on the core of Project Isolation, i'd recommend using FPS Core, which includes 95% of the code that Project Isolation does, but comes with tutorials, documentation, and a support community, and a lot of bug fixes and improvements that haven't been backported to Project Isolation.
A note on licensing - everything uploaded to the Project Isolation repository is licensed under an MIT License, this means that you can use any and all of it however you see fit, without my explicit approval, and you don't owe me anything for using it. Your projects can be personal, educational, commercial, whatever you want. More on the MIT license here.