Is there a way to convert built Unity game back into a project?

No, you cannot unpack a Unity build into the source files for the project.

You can decompile the managed assemblies to get equivalent C# code out of them, but it won't be exactly the code you wrote. It will likely be mangled and harder to read, lacking comments etc.

You can try to extract assets like textures too, but you won't get your original files back. In order to make them fast to load and efficient to display on graphics hardware, Unity converts files to GPU friendly formats and applies various compression strategies to them as part of the build process. This can be lossy — the texture you get at the end is not necessarily a pixel-for-pixel match with the input image.

If you have one specific script you don't remember how to write, or one specific image asset you can't re-make, then these strategies might be worthwhile to get a starting point you can then edit by hand back to what you want it to be.

But for most purposes, you'll be better off remaking the project from scratch, using what you learned the first time around to do it better this time.

One such lesson should be to use a version control system and keep a backup somewhere so you never land in this situation again.

Yes, you can. Try programs like AssetRipper (which's a fork of uTinyRipper program).

How well these programs cope with unpacking depends on the unity version, the older it is, the better.

Yes there are tool nowadays like assetripper and some mono assembly decompiler but the problems will soon rise when your trying to decompile the asset bundles and shader which is currently possible but very far to be stable to be used on your decompile project and you may need on expertise on rebuilding the shader based on my experience may differ on later unity builds

No, you cannot directly convert a built Unity game back into a Unity project. However, it is possible to decompile the game's code. When a Unity game is built, the C# scripts are compiled into .NET assemblies, which can be decompiled back into readable C# code using tools like ILSpy or dotPeek. Once decompiled, the code can be modified and recompiled, but this process won’t give you the original Unity project structure (scenes, assets, settings, etc.), only the scripts.

Bear in mind that proper games often implement security measures, such as integrity checks or hash verifications, to detect changes in the code and prevent tampering. However, some games don’t have these protections, making it easier to alter the code.

Another approach to modify games is using tools like Cheat Engine, which allows you to read and modify memory addresses while the game is running. This can affect variables like health, currency, or other in-game values.

That said, I would caution against doing this. Depending on your goal, it could involve unethical behavior, such as cheating or attempting to reverse-engineer and reuse someone else's work without permission. This would likely violate the game's terms of service and could even be illegal, especially if you're using someone else's code to build your own project.

In short, while it's technically possible to decompile and modify Unity games, you should consider the ethical and legal implications before proceeding.