[elcomet]

Your smartphone does much less, not more.

When you are casting your smartphone on a built-in tv, the TV is doing all the work (downloading and decoding the signal). Your phone is just a fancy remote control.

The Chromecast is downloading the video and sending it to the TV on the contrary.

[kalleboo]

The person you're replying to is talking about playing the video directly on the phone, not casting.

An iPhone 14 Pro Max can play streamed video on the internal display for literally 25 hours on a single charge.

edit: the 14 Pro Max has a 17 Wh battery, which means it draws 0.7 W streaming and displaying video.