Hello! I see Agora and Jitsi mentioned, and other third-party providers include Twilio, Vonage, and Daily (full disclosure: I work at Daily).
While you could build a WebRTC application yourself, there are a few limitations that third-party providers take care of:
- Cross-device issues
- Network bandwidth and track/encoding configurations for more than 1:1 calls
- Scaling
But, if you want to build yourself, I find this repository really useful: GitHub - openrtc-io/awesome-webrtc: A curated list of awesome WebRTC modules and resources.
I hope this helps!