It depends on the company for how much any developer role will be responsible for, but a backend developer would typically be responsible for anything that isn't the user interface. I'm unsure how often the web app statement is true, honestly.

As a lead backend developer I'm responsible for the overall scalability of the platform I work on. Scalability ties to performance, throughput, and maintainability. In my specific position, I tend to leave performance of the code to the members of my team who are implementing a feature while I am building designs that focus more on throughput and maintainability.

This obviously requires a knowledge base of the language I'm working in, but also various mechanisms for distributing work across multiple services and locating bottlenecks in a platform to target for improvement.

I currently work in a combination of Python and Rust. My last position was Golang. Cloud infrastructure comes in to play quite often. AWS knowledge will be really useful in this field. Some relationship database knowledge will go a long way. Learning how to properly build and use a cache.

Overall, I would say it's not repetitive in the long term. Early on it might be a lot of hammering out other people's designs but you get a greater degree of flexibility as you progress and learn more.