That in turn implies that there always is interesting work in startups, doesn't it? On the flip side, there is "interesting work" that only happens in large com

That in turn implies that there always is interesting work in startups, doesn't it? On the flip side, there is "interesting work" that only happens in large companies. If you want to orient yourself towards that, you have to join one. In my experience, strong enough motivation and starting to work with the right teams can get you there. This begins with applying for the right positions. But even then I'm not talking about any "grand plan", just about naturally gravitating towards the work that interests you over time. It may take a while, but by the time you're fully there, you've got experience (and de facto worked on it) already.