ILSpy edits wont save .dll

(I’m on mac btw) So I want to try game hacking, and I’ve done some research. So I made a simple unity first-person shooter and built it, and I went into the files and found Assembly-Csharp.dll and I move it onto my desktop and then I opened it in vs code with the ILSpy extension installed and ran the decompile on it and boom, I see my functions, with my code. So I try to modify some code to make it play a different gun shoot sound, and I hit save, and then I move the file back to where it was (Desktop/mygame/contents/resources/data/managed/Assembly-Csharp.dll) but when I run it there’s no change. and if I don’t move it back but just re-decompile with ILSpy then the changes arent there. What am I doing wrong? Sorry if I’m just making a dumb mistake I’m new to this kind of work. Thanks!