I use WhatsApp as a means to keep in touch with friends and family. While I am on Android unfortunately some of my contacts use IOS :-(
On Android I just leave the app running (I am sure once I open it and even if I move to another app it's still running) and rely on notifications to let let me know if a message has arrived. To avoid notification noise I have the tone turned off apart from one or two very special people for whom I have assigned a recognisable tone. For the rest I rely on an icon in the status bar.
But I notice for those receipients using IOS I will see two ticks (light colour) when a message has been sent and received, two ticks dark colour when it's been read which is normal. But often I will see only one tick which means message sent but not received. Most of my recipients don't turn their phones off so if they have used WhatsApp during the day, is the app still running in the background or when you move to another app it shuts down? I can't imagine my friends actually shutting down the app after reading and responding to messages.