Whatslog on Android allows you to check the online status of whatsapp users. You don’t need to scan a whatsapp qrcode, you just have to enter a phone number and it works pretty well.
I don’t understand how they do it.
I’m pretty sure they don’t use web.whatsapp.com because they don’t ask any QR code furthermore I have reverse-engineered the apk to java with jadx and I don’t see any trace of them using any “hidden” whatsapp api or anything like that.
I don’t see any communication between Whatslog and Whatsapp through Android Intents or the like.
There is this question which discusses the same thing but the answers talk about web.whatsapp but I’m pretty sure it’s not what they use : How can application like Whatsdog detect user online status from Whatsapp?