Have a look at NextCloud. I think it’s FOSS so you can self-host and might be able to tap into the API - it does have an official app already, but you could build one off of that as it has video calling.
It’s pretty good and must be secure as it looks like the German Federal Government (or at least part of it) uses NextCloud (presumably instead of Office365).
Alternatively not sure if you can use the canvas somehow? But I’m sure your best bet is to use WebRTC like StartShare said, but with Firebase?
For example: https://websitebeaver.com/insanely-simple-webrtc-video-chat-using-firebase-with-codepen-demo
…Could even build in Auth via Firebase as well?
Oh and there’s always the free Google Meet, the Enterprise version Netflix execs use for their payment talks, so that’s got to be pretty secure! You can have an app that creates Meet links via API and shares them or something?