Streaming Quality
Re Apple TV (A1842) - does the Video format selected in Settings determine the streaming quality of shows watched via Netflix, Amazon Prime etc? eg if 720p SDR is selected, will Amazon Prime stream in SD?
Apple TV 4K
Netflix will stream according to your Netflix plan, up to the settings as set for your device, but not higher than that.
Amazon Prime Video will stream up to the settings as set for your device, but not higher than that.
720p and 1080p are both considered HD, not SD. SD is 480p/576p.
If no 720p stream is available, then a 1080p stream may get downscaled with that setting.