In my opinion, there is absolutely no way a junior / SWE1 could greenfield an app by doing anything other than following a youtube tutorial or similar step by step guide, and would have basically no understanding of the decisions made throughout. They would have no real experience with style, linting, no good opinion on libraries, no knowledge of accessibility, security, usability, nothing. They would end up with a basic 'hello world' application that would likely fall apart before you could even get your business logic in there.

If you have a SWE1 that can literally just greenfield an entire application and do it at a very high level, then they are a unicorn / 10X developer operating at a ~SDE4 level already.

I personally do not think the average SDE1 can meaningfully contribute to a quickly moving and efficient team. By the time they are even able to contribute regularly, they are SWE2/mid in my book.

I do have much less experience in giant beaurocratic slow moving organizations so I don't have an opinion on how all of that rigmarole works, I'm speaking more to highly focused small teams delivering at a high level quickly.