Do mobile dev jobs involve back end work?

It depends on your job description. Usually Android/iOS developers do not need to work on backend/server-side/api end work. But if your company is a small team and everyone wear different hats, you might need to handle some of the backend services.

I am a front-end web developer at my current job, I have seen my co-workers (Android/iOS) never have to touch backend code, we have dedicated team for that. But in my side projects and/or product that I am building full time, I handle backend, front-end, DevOps, and all other technical stuff!