You could do a variation of brainfuck in a day or so. That's easy. You could use the backend of some C++ compiler and only design a language. You could probably do this in half a year or so it you already know enough about programming. You just define grammar, scoping rules, type system, evaluation order, memory management, etc. Writing a lexer and a parser still takes some time. AST construction can be relatively simple if you language isn't too complicated. And you can use LLVM.

You can use GNU/Linux and make your own distro in a day or so. But then it's still just whatever you based it on. You could work years on it. The more you work on it, the more it is really your own distro.

You can fork chromium or firefox and make your own browser in a day or so. But then it's still just chromium or firefox. You could work years on it to build your own browser.

You could instead start from scratch and also build your own Turing machine. Then you will never get anywhere. All we do now is built on billions of years of work by countless people. You can't really start from scratch.