Mobile companies offer two basic services: mobile voice service and data transmission. Mobile voice service is provided by a wireless connection to the transport towers, and is supported by the mobile switched network on the carrier side, which connects to the public switched telephone network (ie telephone service). SMS is part of this voting standard and allows short messages to be sent over the telephone network, and was very popular before the rise of smart phones and fast data connections.
Your mobile phone, whether smart or not, as chips and radios that support these frequencies, connect to voice calls and other voice services.
The mobile operator also provides data services, which are designed to carry network traffic such as the Internet. You've heard of things like "3G" "4G" "LTE" "5G" referring to these faster and faster data connections. (Confusingly, some of these include voice and data, but let's keep it simple and just consider these data "connections for now." These may use a different set of signals and radios than the voice service mentioned earlier. and data connections, although some older phones only support voice.
So, the best way to think of this is that an old-school phone call uses the telephone network, which counts as "minutes," and all other applications use the computer network, and it counts in gigabytes (GB), which is why Whatsapp, which is an app, data or Gigabyte, doesn't take minutes.
Okay, let's try not to get confused: Let's say you don't have any apps on it brand new phone. You give your mom or dad your new phone number and they call you. That app, the one who calls when they call, is what counts as minutes, EVERYTHING OR on your phone counts as d ata or gigabytes.
Yes, you can call e a voice signal over Whatsapp or Facetime or many other apps. But they use Gigabyte to send a voice call over the data network. They do not count your minutes because they do not use the speaker network. It's best to think about this with the app: it's an app on your smartphone that counts minutes, and it's the phone app. Everything else counts gigabytes (data).
Wifi uses a different network to support the data page of the phone: when you are on Wifi, the phone is connected to the local Wifi router, NOT the carrier data network. Therefore, because your phone uses Wifi, rather than the carrier's computer network, you do not have gigabyte costs. However, because Wifi only replaces the data connection with your carrier, the voice radios on the phone connect you to the carrier. So regular phone calls continue to work on the phone app, and Whatsapp does too. Only Whatsapp uses Wifi now, not transport data.