The Test part of TDD isn’t meant to encompass your whole need before developing the application. It’s function-by function based. It also forces you to not have giant functions. Let’s say you’re making a compiler. First you need to parse text. Idk what language structure we are doing yet but first we need to tokenize our steam. You write a test that inputs hello world into your tokenizer then expects two tokens back. You start implementing your tokenizer. Repeat for parser. Then you realize you need to tokenize numbers too. So you go back and make a token test for numbers.

So you don’t need to make all the tests ahead of time. You just expand at the smallest test possible.