Whatsapp tracker apps?

I was surprised to hear that there were such things like whatsapp tracker apps.
These apps can notify when someone is online and tracks there activity.

i wondering if there is a opensource/foss app that can do this?

There’s no such thing…

Well, there might be some mods. But, I will never recommend you to install them, they will destroy your privacy. They can inject malicious code in the modified version. I personally don’t use them. Not recommended at all.