MainButton.js
import {View, Text, Stylesheet, TouchableOpacity}


const MainButton = props => {
  return (
    <TouchableOpacity onPress{props.onPress}>
    <View style={styles.button}> 
    <Text style={styles.buttonText}> {props.children} </Text>
    </View>
    </TouchableOpacity>
  )
  
}
const styles = StyleSheet.create({
  button: {
    styles etc
}
  buttonText:{
  styles etc
}
})

export default MainButton

//Different file
import MainButton from "FILE-LOCATION"

<MainButton onpress={ADD-FUNCTION-LOGIC}> 
  START GAME
</MainButton>