In the past, some programs such as WhatsApp Sniffer that allow spying on WhatsApp conversations when using a WiFi network became very popular, since messages traveled over the network in plain text and could be intercepted by sniffing the network.
But this problem was solved in 2012, since then WhatsApp messages come out of the phones in encrypted form, so they can no longer be captured and viewed. Even today, you can find many applications that claim to be able to hack WhatsApp to spy on it, but they are all fake and only seek to trick users into installing Trojans or getting money.
Remotely, that is, from a distance, there is no way to read someone else's conversations. For this to happen, which should be clarified that it is a crime, you must have physical access to the cell phone or have the person install a Trojan. The following I comment simply for information because the idea is not to teach how to do it, but to comment on it to avoid it happening to us.
Read WhatsApp with physical access: Besides obviously seeing the conversations directly, someone could copy the WhatsApp database and then read all the saved messages and even some of those that were deleted. Once they copy the file, they could also get it from some backup, they can use a service like recovermessages.com to view messages easily.
Another method very used nowadays is to enable the WhatsApp web version (https://web.whatsapp.com) with which you only have to read a QR code from your cell phone, the web version can be opened from a desktop PC and also from another cell phone through the browser.
They could also steal the entire account by installing WhatsApp on another device and confirming the mobile number with the code received by SMS. With the message preview enabled, they could still see that code with the screen locked.
Read WhatsApp with a Trojan: They could also be installed in a matter of minutes, some are commercial and offer everything prepared so that a person with minimal knowledge can install and configure it. One of the most famous, undetectable and compatible with all mobile phones was StealthGenie, but it no longer works and in fact its creator was recently arrested by the authorities. However, there are other similar ones, so you have to be careful when the cell phone is left in other hands …
We must also be careful with the applications that are installed, if they do not install the Trojan manually, they could do it with social engineering through some false or malicious application that can even be easily sneaked into Google Play.
There are other tricks that, although they do not allow you to see the conversations, can be interesting from the point of view of privacy. If they know our number and add us as a contact, without exchanging messages they could see our profile picture, status and connection times, although this will depend on the privacy options configured.
They could even get our IP to try to geolocate us if they send us a message with a link to a page, as I did with the Nigerian scammer some time ago.
In summary, you have to be careful with the applications that are installed and in which hands the cell phone is left. And of course, nothing to believe in those spam programs, because they do not work and only seek to infect unsuspecting users.