WhatsApp, Instagram, Telegram and the like don't exist as an app on any smartwatch. You can just use your smartwatch to read notifications and, in most cases, answer them.
You can get an apps called informer which allows you to push messages almost at will, and not simply reply to them using your watch, and this apps relays them thru the apps like WhatsApp and telegram on your phone. It costs a few quid. You can also sideload whatsapp although it is a bit of a process and not quite as easy to use as informer. There are a few youtube vids on this subject if you have a look
