There are many examples for creating real-time collaborative whiteboard drawing in web, if i want to do it in android which should work with iOS app also. most of the links which i saw is web-based:
https://github.com/JohnMcLear/draw
https://github.com/mirceageorgescu/Real-time-canvas
https://github.com/byrichardpowell/draw
https://github.com/cshum/aerosketch
https://github.com/MikeMcQuaid/Whiteboard
https://github.com/gipsyblues/Go-Drawingboard
like this there are lots of projects based on web,webrtc,nodejs and loading webview in android. If i want to do in native android from where i have to start.