If you use a regular webpage then they just need to use Safari mobile on iOS for it to work I believe.  On Android you can use pretty much any browser (Firefox Mobile, Chrome Mobile etc).
If you do not need the “Native app” then this would be my preference as web apps are just as powerful for many things these days. Also consider PWAs:

  
      

      developer.mozilla.org
  

  
    

Progressive web apps (PWAs) | MDN

  Progressive Web Apps (PWAs) are web apps that use service workers, manifests, and other web-platform features in combination with progressive enhancement to give users an experience on par with native apps.


  

  
    
    
  

  


But you would need to check support on iOS for that.
If you do need “native app” then your best bet is React Native that I linked above.