Is it only because of ad campaigns, or is there something else? For example, I’ve read about organic growth. Is it actually possible to grow that way now?

I thought organic growth meant that you basically do nothing, and the app or any other product is just freely floating on its own.

It’s always a combination of several things. Organic traffic, ASO, advertising, reviews, and audience retention. Maybe I forgot to mention something else. Simply releasing an app, even a really good one, is no longer enough. For working with organic growth and keywords, I recommend https://asomobile.net/en/. In a highly competitive situation, it’s super important to make the app noticeable and make sure users actually come across it.