It's really no big deal, and you'll figure it out easily.
Zoom works through a web platform, they'll send you a link via email or text that you click on, it takes you to the platform, you enable your microphone and camera and then the call starts, it's easy peasy.
Facetime has its own app on the iphone, it's like a video calling service between iphone users (I think only iphone but i'm not 100% sure).
Both involve each user having a camera and their mic on, sometimes with zoom if the connection is bad you might disable your camera to use less data.
Regardless of whichever you have, if you have wifi, you can disable your phone's data connection and you won't use a penny of your phone allotment (most plans will give you minutes of telephone calls and then a quantity of data, usually measured in gigs or GB, and then maybe a number of text messages).
It sounds confusing but you'll catch on super quick.
