And how can somebody become a senior dev unless they first worked as a junior

Perhaps some juniors will take it head on solo but will repeat the same mistakes seniors made when they were juniors but will survive nonetheless

You become a 'senior dev" by running a company / startup yourself, since it is clear that after the tech layoffs, almost no-one has the money or profits to hire any new developers - junior and senior, which is why I say they are both affected.

But even then, self-proclaimed seniors are too scared to start their own startup(s) now because of (1) Unfavourable market conditions (2) VCs hesitant to raise money (3) ChatGPT will extinguish their startup; even if it uses "AI".

I guess this was the result of a decades long quantitive easing, near zero interest rate bubble of cheap money that had to collapse.

I don't think starting a company will be that successful if you lack enough experience to be hired at any existing companies. The survivorship bias in tech is huge. Most things fail.