One can do note taking with self on Whatsapp.
Quoting from whatsapp website (https://faq.whatsapp.com/5913398998672934)
> Use https://wa.me/<number> where the <number> is a full phone number in international format.
Put your own number and you chat with yourself. Pin it to top, so it's always there. I use it to add information, search later use cases.