Will I consume less data if I cast a YouTube video from my Android phone to the TV rather than using the YouTube app in the Smart TV?
Both your phone and TV need to be on the same wifi/network. TV will consume data, phone will be minimal, just the thumbnails.
The TV will consume the data bit…
No you won’t. When you cast, the TV optimization begins, meaning it will play like it would have played on TV in the first place. But I don’t imagine you will be tethering your TV via mobile data unless you have loads of that.