The good news is that Freevee is an App provided by Amazon that is available for a number of platforms, including Android, to provide a subset of many Amazon Prime films that are FREE to watch, but supported by their Adverts.
The bad news is that at present there is no easy way to 'cast' these films from the App to a large screen, like a TV with a Google Chromecast with TV [or other device] active in it's HDMI port.
You can use the Freevee App to discover films that you want to watch then save them in an area accessed via the App called "My List".
Click here to visit the Amazon Portal that is positioned at a page following a "Freevee search" displaying a selection of 8 Freevee videos that they are currently promoting. However if you know the names of other Amazon Prime films [that you may have saved in "My List" using the Freevee App] then you can search and locate these instead.
First you need to open the Amazon Portal using the Chrome desktop browser which supports 'casting'. Next select the film you want to 'cast' - then in the top right corner of your browser click on the "3 dot menu" option for "Save and share" and select "Cast" at the bottom of the list - select the Chromecast device to be used. e.g. Lounge TV [named CCwithGTV]