Plenty of devs work on both web and mobile in their day-to-day, or do web development at work then go home and work on a mobile app. Unfortunately most companies separate the two into different teams with zero cross-collaboration, so frontend devs get no opportunity to work on the app or vice versa, but that's the organization's fault, not the developers'. Same with frontend and backend. If you're capable of learning more than one language/framework, you're capable of doing both web and mobile. The rest is mostly just grasping differences in UX design (and owning a Mac). Don't limit yourself just because your colleagues can only do one thing. Most people simply never have the opportunity or interest to practice outside their specialty.

Depends on ecosystem (work, team, and tasks)