How does real-time video calling work in apps?

There are two separate problems. Signalling is how two clients discover each other and agree on codecs and addresses, which runs over your own server. Media is the audio and video itself, which tries to travel directly between the participants. When a direct path can't be established — which is common behind corporate firewalls and some mobile networks — the media has to be relayed through a server instead.

That relay is where the surprises live. It costs real bandwidth, it needs to be provisioned near your users, and the fraction of calls needing it is impossible to know until you have real users on real networks. Add the mobile cases — a call surviving a switch from wifi to mobile data, or the app being backgrounded mid-call — and this is one of the few areas where I would reach for a managed service before building.