1. Cable provider is Xfinity.
2. Xfinity gateway connects to Roku Ultra Wirelessly, and Roku Ultra connects to 4K LG TV through HDMI cable (because video and audio needs to be fed into the home speaker system).
3. The GOAL: Is to reduce the data usage to stay within the Xfinity Data plan
a. If the Roku ultra player is set at 720P (and the youtube video can display 240P through 1080P)
i. If the Youtube video is set to 240P, will I use less data?
ii. or, because Roku is set to 720P, all I'm doing is using the same data BUT suffering a lower quality video?
Your data usage will be determined by what is FED to the Roku, so your Your YouTube settings should prevail.
To minimize data usage be sure you return your Roku its Home screen when you've stopped viewing. Otherwise your Roku can continue to stream in the background even though you've shut down your TV.
Also you can turn on the "Bandwidth saver" to shut down the Roku when it's received no user input in 4 hours (Settings > Network > Bandwidth saver)..