Yeah, that line’s a real gray zone. Public domain code is free to use, but most open source is under licenses that still give you rights to copy and modify. Once you mix proprietary code or add your own significant changes, you can claim ownership of that new work, but you still owe the original license. It’s all about the license terms, not the file itself. So the blur is where the license says you can do anything versus where it imposes conditions. It’s a good angle to dig into.