https://www.androidauthority.com/samsung-galaxy-chromecast-one-ui-6-3356571/
Samsung is making it easier for your Galaxy smartphone to cast content via Chromecast with the latest One UI 6 update. Read on to know more!
I have samsung phones and chromecast devices and never had issue casting. However I believe you need the google home app for it. If you install VLC that might have its own library in the package for casting as well the youtube app. Don't forget most TVs have miracast so if you are using the screen share option with your TV that is not chromecast.
You can cast from any app that supports it. I think the article talks about screen mirroring with chromecast.
I just cast from whichever app I was using. For instance, I have the HBO app on my phone, I open it, find what I want to watch, and cast from there. I think the change is that you can cast your screen now whereas previously, I only was able to use screensharing with my Samsung TV.
This only highlights and strengthens my decision to never buy a Samsung phone. They so desperately want to go their own way but don't quite have the guts.