Good question, I would say neither, this responsibility is for the browser and native apps you use which display web content (e.g. Chrome, Firefox, Facebook, etc). Not apple or the web app developer. At most OP should include https://www.npmjs.com/package/webrtc-adapter as part of their polyfills to aid cross-browser support/cater for WebRTC nuances.
WebRTC has been in iOS Safari for quite a while. However, all other apps and browsers were limited to what was in offer within WKWebView. From iOS 14.3 beta 1 onwards WKWebView gained getUserMedia/WebRTC features. It is now up to the browser and apps you use to make sure they have implemented their web views correctly and allow the correct permissions for WebRTC to work.
I wasn't able to test much on iOS unfortunately. I know on Windows 10 if one browser is already using the camera it will say camera unavailable when launching a different browser. I am not sure if it is related to this?
