Screen mirroring is NOT casting. Whatever you call these things, they're not the same. Screen mirroring is that sluggish "feature" that nobody really needs, which is projecting a smartphone screen onto a TV. It's cumbersome, slow, prone to projection (as in display geometry) erros, etc. It's a presentation thing for a projector, not a thing you'd use to show off your Google Photos on a TV.
Casting (as in Google Chromecast) on the other hand is sort of letting the app on a phone forward it's linked contents to the receiving app on the TV and as soon as the app picks up the "stream", it "takes over", "freeing" your phone (which can still control the stream, as in "play", "forward", "reverse", etc.) The most obvious difference is that as soon as the stream is passed to the device (a TV), the TV is rendering the stream, NOT the phone, which in case of the screen mirroring, would be the renderer and the TV would only receive frames to show on screen.
"Casting" is the way to go if you want to pass that YT stream from the phone to the TV, or HBO or whatever app there is which supports it. It's also the fastest way to show off your Google photos and videos on a big screen.
"Mirroring" is useful when you want to make a presentation of your phone screen and whatever things you do on your phone. It's NOT the way to show photos or videos on a TV screen.
That being said, Samsung TVs don't have a clue about casting. All they know is mirroring. Unless you AirPlay with iPhone. That means the only way to make (new) Samsung screens let you CAST is to get Google Chromecast or switch to Apple. Why "new" you ask. Older (2017) Sammy TV sets supported Samsung Smart View app which let you cast. Newer TV sets do not support it, handing over to Samsung Smart Things "thing". This app, however, does not support casting, for whatever reason. The "logic" behind this is surely disfunctional, but hey, it's how things work.
