Coder question: Is there a Chromecast open source project?

[Cebolla]

If it's an Android phone, you may be able to use Share on those apps with the BubbleUPnP app & thus get similar direct streaming from the server to a UPnP renderer:

https://bubblesoftapps.com/bubbleupnp/tips2.html#other_apps

 

I believe there hasn't been a working unofficial Chromecast player since Google made a major security change to the Google Cast API a few years back, eg:

https://github.com/dz0ny/leapcast