SmartifyOS is a base application (source code) that makes it easy for you to create a custom GUI and 3D render for a DIY infotainment system in older cars. It uses the Unity Game Engine, which means you have almost unlimited possibilities to customize it to your liking and that there are a ton of tutorials for almost everything online.
SmartifyOS gives you the ability to edit the full source code to customize everything to your liking inside the Unity Editor. If you're done customizing, you can use one of the included GUI Editor tools to export an installer for Linux Debian with LXDE. You can then run this installer on your mini computer of choice (only x86 at the moment), and it will install and set up everything for you (you need an internet connection for the first install). After you did the first install, you can use the option to export as an update (onto a USB drive).
Yes, SmartifyOS works for every vehicle. It even has an online library of cars that can be imported with one click. But even if your car isn't in there, you can use your own 3D model (and also upload it for others to use).
To use SmartifyOS you don’t really need a lot of coding experience since it has GUI tools for the Unity Editor, and for everything that is more complicated, like communication with serial devices (Arduino), SmartifyOS has easy-to-use and fully documented APIs (maybe later some YouTube tutorials too). You also don't need to make everything from scratch (everything on top the base app) because it comes with a fully working system included (for a Mazda MX-5 NA). So you can only modify some little things you want to have different, and you can also look at it to see how everything works. But you can, of course, also delete this and build it yourself.
Yes, SmartifyOS is free for personal/non-commercial use (for more details have a look at the license on GitHub) and you can always access the full source code on GitHub.