This was tested using Ubuntu 18.04+

There are several ways of sharing and streaming using either Chromecast or Chromecasr Ultras:

1. Using Google Chrome with the Videostream App)


2. Using SMPlayer or VLC, both of which have the option to Stream to Chromecast

SMPlayer

VLC

3. Using Youtube (In Google Chrome):

This helps a lot when you are managing multiple rooms, kids, NetFlix accounts and more. Do note that if Chromecast is not detected, 99% of the time is the network configuration. You can find a lot of helpful advice at the Chromecast Help Page or going to the Troubleshooting Guide.

You can also share your desktop by using the Cast option in Google Chrome:

And after selecting any of the Chromecasts you would see something like this: