I just got to college and can do web dev fairly well (Mern stack mostly). But tbh I am now more interested in being an app developer. Should I try to learn React Native/Flutter or Kotlin/Swift

I'd go for Kotlin personally. But it depends really, I'd try and work out what kinda app i was gonna make and find out if there exists a framework for some language that'd help me out and pick from there.

Flutter is worth your time too, it has best of both worlds, something React Native is missing

It depends what kind of apps you want to make. You can keep doing JS based react native stuff if you want to do things cross platform. You can even make PWAs and sidestep app stores.

If you want to make something like a game then you could do all sorts of other things like learning unity or etc.

Either you want to be paid to do it or you don’t. If you don’t want to be paid you should just do what feels right. If you want to be paid you should learn the skills for that industry.