[aeyes]

But lots of people here talk positive about services like Heroku or Fly where you just push the code somewhere and it runs without you having to know a lot about the infrastructure.

Not every software development problem is a big scale problem and once you identify such a case you can start optimization work taking all the low level details into account. In reality most scalability problems revolve around databases, caches, concurrency and locks and you probably aren't going to tackle a lot of these in your average stateless service.