I would like to search for Emojis using text when using an Emoji keyboard on iPhone. I have seen some people using this (also, I guess WhatsApp keyboard supports it?), and was wondering how to enable this, does anyone have a clue?
You can't use search when looking for emoji in the stock iOS keyboard.
iOS uses predictive text to determine what emoji to offer you based on the words you're typing. While this isn't what you're asking for, you can see this iPhoneLife article for more info.
To search by emoji, you need to install and use a 3rd-party keyboard. Many developers offer keyboards that incorporate emoji search. I use SwiftKey (not associated with the software or developer) for typing on all my iOS devices.