I’m trying to share my screen on an iphone in a webrtc call— anyone know how I could do that?
So you can’t really do that today. This is a browser-level limitation.
Check out this getDisplayMedia() docs. The browser compatibility section at the bottom shows what devices and browser combos allow you to use getDisplayMedia(): https://developer.mozilla.org/en-US/docs/Web/API/MediaDevices/getDisplayMedia#browser_compatibility
I always go here: https://caniuse.com/mdn-api_mediadevices_getdisplaymedia
or go here from the device and browser where you are trying to screenshare: https://webrtc.github.io/samples/src/content/getusermedia/getdisplaymedia/