https://lucstechblog.blogspot.com/2019/ ... o-esp.html
This uses Mit's app inventor to build an app and ESP-Basic to send data.
If you NEED to use Arduino lingo (C++). Look into building an ESP web-server.
On the webpage you can display status of switches, temperature etc.
But then you are not using an APP but just a simple webpage which can be seen by any browser.
If you need to work outisde your local net then you could check MQTT and use a web-based
broker on which your app checks every x minutes and the ESP sends its data to.
Another option is using Dweet.
Send data with your ESP to Dweet:
http://lucstechblog.blogspot.com/2019/1 ... duino.html
Receiving Dweets with Android:
https://lucstechblog.blogspot.com/2019/ ... droid.html
Many solutions available. Google is your friend.