It is free. You can create a video link by using a random text at the end of the URL.
For example:
The key point here is when you create a video call between the 2 users, give them the same link. Anyone with the same link can join the conversation. The rest is up to you on how to figure out the creation of the links automatically.
This calling works in a webview component. So if you set a screen with a webview component and add the jitsi link, it will work within the app
The paid version requires custom development but it also allows you more flexibility.