Ask HN: How would you build a Phone app (iOS,Android) in 2019?

I work with web apps for a living but never really got into the whole app thing yet. Recently, a few clients have come up and asking about apps. What technology would you use today to build it as I know that there are things like react native etc. Also, for phone apps, is there a specific server side language that is preferred for any specific reason or any good server side API based would do ? Lets say that the app would be used by about 5000 users in a month (nothing big scale wise).